Tube rolling thread | UltraSonic Studios

[OctavianH]

Tube rolling thread | UItraSonic Studios
(for amplifiers with tube rolling add-on and different custom features)

(Infinity with tube rolling add-on | UltraSonic Studios, picture taken from here, all credits to @SonicTrance, specs at point 1) below)

On 28.02.2023, Lars Tomas Jernberg, the mastermind of UltraSonic Studios, died. This means no more new builds will be added. This thread will remain active as long as current owners will continue to use the existing models.

Version:
15.04.2023 - 2.1 - added remark related to Tomas's death
12.11.2022 - 2.0 - added the Push-Pull version of Odyssey
15.06.2022 - 1.9 - added a remark about the black version of Odyssey and Finissey (Odyssey and Infinity combination)
17.03.2022 - 1.8 - removed Tube Compatibility Guide since no other manual grid bias amps were built and can be misleading
22.10.2021 - 1.7 - added some useful links in the Resources section (including a russian equivalence table)
05.04.2021 - 1.6 - updated Eternity specs and tube rolling procedure
14.03.2021 - 1.5 - updated the Tube Compatibility Guide with the values from here
11.02.2021 - 1.4 - added Odyssey classic SET amplifier
07.02.2021 - 1.3 - added Tube Compatibility Guide section
02.02.2021 - 1.2 - added Resources section
28.01.2021 - 1.1 - added methods to calculate grid bias value (preliminary)
26.01.2021 - 1.0 - initial release

Introduction

This thread is supporting the Tube Rolling add-on available on Infinity and Eternity tube amplifiers, as an additional resource for the original UltraSonic Studios thread. The purpose of this one is to extend the discussions on the tube amplifiers built by @SonicTrance on behalf of UltraSonic Studios without mixing them with the original builder's thread where the focus is the stock version of the amplifiers. Besides Infinity and Eternity this thread will support also Odyssey which will be a classic SET amplifier. The following tube rolling principle will not be valid for Odyssey, which will have automatic cathode bias.

General principle of the tube rolling add-on

When UltraSonic Studios released Infinity, following the Oblivion and Citadel builds described in the builder thread, more and more people started to ask about specific custom builds on other tube types than the ones used in the stock version. The philosophy of UltraSonic Studios is to always use cheap tubes in a design which will provide sound quality above the price. However, several users on these forums, including myself, owning already several tube types wanted to use them in the upcoming amplifiers. This is how the tube rolling add-on appeared.

The tube rolling principle is based on variable Grid bias voltage (multiturn trimmers) for each stage (input and output) of the amplifier. By setting a specific negative voltage for each stage, you are allowing the amplifier to accept different tube types with different electrical parameters. In order to allow the setting of these voltages, the builder will add measuring points (banana jacks) on the top plate of the amplifier where, using a digital multimeter, the owner will be able to measure the voltage for the input and output stage and adjust it to match expected tube type bias. There will be three points for both stages: two positive (one for input and one for output) and one common (ground). Besides the measuring points, the amplifiers will have mA meters to measure the current flowing in each tube or stage (depending on design you might have a common mA meter for both input/output tubes or a meter per each tube, these details have to be clarified with the builder). In case you will have only one meter/stage you will be able to opt for a switch to commute between channels and see both currents on the same meter based on the selected channel. These meters will provide complete control and verification of the bias is setting and also provide information about tube aging (here we will update when more info is available). More details about this feature in the following post, the first part of the Q & A session with the builder.

Disclaimer

I am not the owner of all this information, this belongs to UltraSonic Studios and the people who contributed. I am just gathering and updating the first posts of the thread and I will always post the source and give credits to the owner of the information. When this would be not possible I will make a remark about it. Some of the information presented belongs to me and has no other source presented.
I will also update periodically the first 3 posts, reserved for Summary, Links and Important Information which have to be easily found by others. I will also update the specs of the amplifiers, add explanations for add-ons and everything needed. For any suggestion or mistake feel free to write me and I will do my best to keep this accurate and up to date.

Below there are some examples of custom amplifiers which incorporate the Tube Rolling Add-on or even a classic SET amplifier with auto-bias. When more amplifiers will be built and if these will incorporate new features, we will add them here. If you are thinking to order a custom amplifier based on Infinity or Eternity, you can take these examples as baseline for your own build. From this point onwards, for more customizations or questions you can, of course, directly contact the builder.

1) Infinity with custom add-ons (pictures here)

Tube sockets:
- 2 output sockets designed around the 6L6 / EL34 / KT66 series
- 3 input tubes so I can do either one 6SN7 or two 6J5 or others with adapters.

For the EL11, MHLD6 and EL32 adapters I had them designed around a single socket. The best adapter is none but since those need one anyway so I guess there's no point of going with two single adapters instead.

The left knob is to select the speaker and pre-amp outputs and the right knob is the impedance switch.
The speaker/pre-amp switch has three positions: OFF/speaker/pre-amp
The impedance switch has also three positions: OFF/low/high

The amp has three different heater windings. Rather than paralleling all of them I kept them separate like this:
Output tubes: 5.4A
6J5 sockets: 3.8A
6SN7 socket: 2.5A

Other features and add-ons:
* 4-pin XLR headphone output
* RCA pre amp output on rear
* Speaker output on rear
* Dummy RCA jack: (connected to ground)
This was my idea allowing me to use "RCA tuning stick" for increased dampening
* Two small bias adjustment knobs on top plate for grid biases but not plate voltage along with 3 ports for an external DMM volt-meter
* Headphone / speaker switch: one position for headphones and line-out simultaneously, and one position for only speakers
* Impedance switch: 3 Positions: headphone off, low, high
* 4 mA meters (two large on front, two smaller on top plate)
We thought one for each tube is better to specifically detect which tube is faulty rather than one per dual set

Tomas had replaced the trimmer(potentiometer)+voltmeter concept of Infinity with trimmer + dip slot; now the user instead inserts an external voltmeter when adjusting the bias which is necessary to improve performance with tube rolling and prevent damage to the transformers. He explains this was done out of necessity for a no-compromise design and using a digital meter is better to give more accurate reading anyway, and I'm trusting his judgement.

2) Eternity with custom add-ons (pictures here)

Eternity is the smaller brother of Infinity, a low power version built in a smaller chasis (340x305x180mm). It is a headphone only amplifier which will have also the Tube Rolling Add-ons and other specifically custom features asked by the owner. If needed, speaker outputs can be added to this model, see below more about power output and limitations in this specific case.

Tube sockets:
- 1 x 6SN7 as input OR
- 2 x 6J5 as input (wired to allow 6N7G with triodes in parallel)
- 2 x 6V6 as output (allows EL34 natively)

Compatible tubes:
- Input: 6SN7, 6F8G, 6C8G (barely), 6J5/6C5, 6N7G (without adapter), 6N1P, C3G, EL3N again most likely a lot more.
- Output: KT66, KT77, KT88, KT120, KT150, 6550, 7581A, 6V6, EL3N, EL32, EL34, EL81, EL36, EL38 and most likely a lot more.

Other features and add-ons:
- Khozmo attenuator (48 steps stepped attenuator)
- Redesign around different tubes (6V6/6J5/6SN7)
- Bias adjustments (tube rolling add-on, see 1) for details)
- VU meters (a light to guide you into darkness, just for appearance, will have an on/off switch to disable it when not needed)
- Impedance switch (low/high gain mode)
- Current through tube switch/channel (mA meters will show left/right based on setting)
- Input socket switch (a switch to choose between 2x6J5 or 1x6SN7, allows you to populate all 3 input sockets at the same time)
- Fixed bias switches (a switch to turn off tube rolling, when ON the amplifier uses the stock bias settings of 6J5 or 6SN7/KT66)
- EIZZ Teflon sockets (tube rollers shield against wearing)
- XLR inputs (for convenience, the amplifier is SE)
- 4-pin XLR output (for convenience, the amplifier is SE)

If needed, additional to what is mentioned above, speaker outputs can be added (3W into 8 ohms is enough for high efficiency speakers). However, keep in mind that the Speaker/Pre-amp switch will replace the Input switch. Therefore, the conclusion is that not all add-ons can be ordered at the same time and every configuration has to be discussed with the builder.

To have an idea about the power output, below is a table for Eternity:

Besides the above described version of Eternity, there is another version, in a different chassis, with dual 6.3V/12.6V independent heater circuits for inputs and outputs, allowing the use of, for example GU-50, 12L6GT or 12V6GT as outputs and 12SXGT or FDD20 as inputs. I have not created a separate section for this amplifier since it derives from Eternity. This version will have a special wiring on 6j5 sockets to be able to use dual 6N7G and also anode caps for the 6V6 sockets (usable for EL38, EL39).

In the end, just to have an idea about the difference in size, Eternity with its bigger brother, Infinity (this one with stock tubes):

3) Odyssey with custom add-ons (pictures here)

Odyssey will be a classic old school tube kind of design. It will be SET, ie transformer coupled and using good specialised transformers with taps of 8 and 50 ohms for headphones and speakers, Mundorf S/G/O caps and maybe Vishay resistors.

Tube sockets:
- 2 x EL11 (as strapped triodes)
- 1 x 6SL7 (as double triodes) - in this same socket, 6SN7 can also be use. 12au7, 12at7 and 12ax7 can also be use via adapters.
- 2 x KT88, KT77, KT66, KT120, KT150, EL34, EL12 spez via adapters, 6L6GC, 6550, 5881, 807 via adapters.

Tube rectifiers:
- 1 x GZ34 which has a very low voltage drop. Any other rectifiers with a higher voltage drop can be use... that includes GEC U52, Cossor 53ku, GZ37, etc.

Other features and add-ons:
- Sowter transformers
- Speaker out
- XLR inputs (in addition to RCA in)
- 4-pin XLR out (in addition to 1/4 inch out)
- Impedance switch
- Mundorf coupling caps
- Yamamoto sockets

There is one power transformer and 2 x output transformers. A choke is also use. The transformers will have winding taps of 8 and 50 ohms. That should cover speakers and headphones. Additionally depending on price, will add another 2 windings - 100 and 300 ohms. That should cover most headphone range and I'm not using iems on this amp that is for sure. The 3 transformers are mounted to the back of the top chassis plate and they will be naked because naked is good. The choke is installed inside the chassis and will not be seen. In front of the centre power transformer is the GZ34 socket. Flanking the rectifier are the KT88 sockets. In front of the GZ34 socket is the 6SL7 socket. Flanking the 6SL7 socket are the two EL11 sockets. It will be symmetrical in appearance.

I've decided to go with the silver chassis used in Citadel for Odyssey because it will blend in very well with the grey colour of the Telefunken EL11 and EL12 spez tubes. Also the silver / grey Sowter's transformers exposed naked in the silver chassis will be just perfect.

There is also a black version of Odyssey, with an audio-gd theme which can be seen here.

And also a Push-Pull version, as can be seen here:

This latest incarnation of Odyssey has some very special features like adjustable operating point and support external power supply for heaters besides triode/ultralinear modes. More details in the above mentioned link.

At the moment these 3 basic types are the only ones we know about. Based on these, more and more amplifiers were built, one very interesting build being Finyssey which combines Odyssey and Infinity. We did not add it as a separate section because it is a combination of 2 already existing types. We have not added Telemachus 300B model since we are not sure about tube rolling potential and how it will fit in the context of this thread. When more amplifier types will be available, if these will have something new, we will add them and also reorganize the information in a readable form. More to come, so bring your popcorn, take your beer, put your most loved headphones close and read this periodically.

 

[OctavianH]

I had long discussions with @SonicTrance when defining the requirements for my custom Eternity amplifier. Some of the discussions are very useful for other people who think to order or want to understand more about these custom versions which support tube rolling. I saved the discussions and I am trying to present them here in a easy readable form. I will structure this as a Q & A session. All credits go to him, I am just a humble storyteller. I will also update this post with other information I will see on the thread, mentioning, as always, the source and owner.

Q & A regarding UltraSonic amplifiers, tube rolling and other add-ons
available on Infinity and Eternity
​

First of all, how do I tube roll?

Equipment and skills needed:
- digital voltmeter
- screwdriver
- minimum electrical skills to be able to measure a voltage

Tube rolling principle:

Each amplifier will have at least 1 x mA meter to measure the current into each stage (per stage or tube depending on the design). There will be measuring points on every stage where via a digital multimeter the user will be able to measure the grid bias voltage and adjust it for each specific tube type. More on this will be presented later, with pictures. At the moment we can say that we will measure a voltage, receive a table from Tomas with grid bias voltages and then adjust the input and output stages accordingly.

Be warned before starting that the grid bias values for each amplifier are different. This means DO NOT set a voltage for a specific tube type on Eternity if you read it from someone on Infinity. This might damage the amplifier! Always set proper voltages on each stage, and carefully check if those are the correct ones for your own amplifier. More than this, the same tube type can have not only different bias values on different amplifiers but also depending on the stage used. Ex. EL3N on output will not have the same value to set as on intput. Since each amplifier can have different operating points, everything has to be checked for each build in particular.

Grid bias adjustment:

Before starting to make any tube rolling or grid bias adjustment please take your time to study the procedures which need to be followed. These need to be followed carefully in order to ensure proper operation of your amplifier and have been detailed and supervised by the builder himself. In short, when exchanging tubes (with or without grid bias adjustment) some waiting time for the circuit to "settle" is needed. What to do is described at the points A) and B) below. But before going there let's first understand clearly how the grid bias is adjusted. The steps below are corresponding to the step "adjust bias" from the procedure at point B), when adjusting grid bias you need to follow the whole procedure presented there.

Steps for bias adjustment:
- set the digital voltmeter scope to 100V DC (or more)
- connect the digital voltmeter probes to the measuring points on the amplifier (marked red and black - see photo below)
- set the voltage to the desired value via the multiturn trimmers (see photo below) on the top plate *
- when turning clockwise the multiturn trimmer the current in the tube increases, meaning that on the digital voltmeter the negative value will decrease
- follow the display on the digital voltmeter to reach the desired negative voltage

(example, these can have different positions on different builds)

* tolerance for currents in each stage: Input stage: 3mA to 5mA, Output stage: 15mA - 24mA (fuse will blow @ 25mA)

During operation, wait 5 minutes after you power OFF the amplifier before powering ON again, regardless of the reason.
(tube exchange, grid bias change, input switch from single/double triode)​

A) Procedure to exchange the tubes without adjusting the bias

- power OFF
- wait 5 min **
- remove the tubes
- insert new tubes
- power ON

B) Procedure to exchange the tubes when bias adjustment is needed

- power OFF
- wait 5 min **
- remove the tubes ***
- power ON
- adjust bias ****
- power OFF
- wait 5 min **
- insert new tubes
- power ON

** to let the power supply caps drain before inserting the tubes.
*** only for the stage you are adjusting, if you adjust only input state, you need to remove only the tubes on input stage
**** a good practice is to start with a slightly higher negative voltage (i.e. -10.5V instead of -10V) and then lower it, ensuring that the tube does not draw more current than is allowed by the circuitry

Now, when I learned that, what should be the Bias adjustment precision?

On output tubes maybe +-one volt is ok. It's more important on input tubes.
If the bias voltage is too low, the tube will draw excessive current and some components inside the amp will fail.
I should say the tube draws more current the closer you get to 0V. The further below ground you are the less current will go through the tube.

UltraSonic amplifiers in general, will I be able to adjust the sound via tube rolling in this specific circuit and design?

These are not old school style tube amps where the tubes are resistor loaded and have a load line which goes into the non-linear region of the tubes. The tubes are always working in their linear region thanks to the gyrator plate loads. Other design choises makes these amps have fast transient response and transparency. But, all amplification is done by the tubes! That's what tubes do best, amplify voltage. MOSFETs supply current together with the output tubes. That way all components do what they do best! There's no SS sound at all from any of these amps.
My amps don't have "driver tubes", they have "input tubes". The input tube don't drive anything. I have source followers (grid drivers) that drives the output tube grid. So the input tubes function is only for voltage amplification. The output tubes however are working together with it's plate load (gyrator) to supply current to the load. Oblivion has some more tube flavor than Citadel and the upcoming Infinity will have even more. The output stages in Oblivion and Citadel are LTP stages with a CCS tail.
In classic designs the input signal goes to the grid of the driver, then the anode goes to the grid of the output tube and so on. It's the same in Eternity also. The difference is that we have source followers in between that drives the output tube grid. You can read about the grid drivers on the "amps" page on my website.
All tube rolling will have effect on the sound sig. In a classic amp the input tube "drives" the output tube. So one has to account for things like rp and other characteristics of the tube. Those things are only important in the output tubes in my amps. But, the input tube will still affect the sound!

How important are electrically matched pairs (input/output)?

In Infinity you would, technically, benefit from electrically matched pairs. More so than in my balanced amps. That's because in the balanced amps there's a CCS that sets the current. That current is always firm. One tube can still draw more current than the other (in the same channel) but the total current remains the same.
Infinity is single ended and grid biased. That means that for two tubes to draw same amount of current at a fixed bias point they need to be electrically matched. With that said, for you to notice any channel imbalance the tubes needs to be very mismatched.

Do these amplifiers have Soft Start?

My amps doesn't have soft start as it's not needed. In old school amps the tubes are exposed to the full B+ voltage on startup. I use a regulated B+ supply and a firm voltage reference to the plate of the tubes. They'll never see one volt above what they're suppose to.

How important is the heater current on input/output tubes when deciding to use a specific tube type?

Regarding heater current that doesn't matter at all in regards to input or output tube. An input tube is high gain and low current (plate current), output tube is low gain and high plate current. There's a bit more to it but that's basically it. Heater current doesn't matter at all. The lower the better.

What is the Max load allowed on Eternity? (will be updated also for Infinity when I will have this information)

The heater winding is speced at 14 A, so not really a concern.

Is there any difference between XLR inputs on Infinity or Eternity vs Citadel?

If I put XLR inputs on Eternity or Infinity it'll only be for convenience. It'll still be SE, no different than an RCA cable of the same quality. Citadel is balanced from end to end. In that amp you fully utilize the 2 phases per channel a balanced signal has.

What transformers are used in the UltraSonic amplifiers?

I use standard Hammond output transformers and Toroidy power transformers. In Eternity, the Hammonds are all you need as the output is parafeed and they're driven from low impedance.

Why manual grid bias instead of auto bias on Eternity/Infinity? (source here)

Less capacitors in the signal path using grid bias. Even if I'd use cathode bias in Infinity/Eternity you couldn't just roll tubes without adjustment. The gyrator is still set at a specific current.

Is there any headphone impedance limitation or recommended range?

It'll drive any impedance but will have highest power output into low impedance than less and less the higher the impedance (ohms law). Most high impedance headphones doesn't need much power to drive and they present virtually no load to the amp. Impedance matching is important for power but not necessary for SQ. Low impedance output will drive most any headphones.

Does it make sense to do crossmatching on tubes (for a dual triode with unmatched section using a stronger output tube to compensate)?

I'd not recommend compensating a weak input tube with a strong output tube. The circuit is most happy with good strong tubes and that goes for all amps. But, especially Infinity and Eternity.

What are the VU meters measuring on Eternity?

The VU meters measure the output signal. So the more you turn up the volume the more movement on the meters.

What is the Impedance switch effect (Low-Z vs High-Z)?

What the impedance switch does is changing the turns ratio of the OT to get higher output impedance. This creates more distortion but also more power into high impedance headphones.

Resources:

Radiomuseum.org (thanks to @hpamdr)

Frank's electron Tube Data sheets (thanks to @hpamdr)

tubebooks.org (thanks to @triod750)

4Tubes - Tube Data Sheets

JACMusic - The KT88 / KT66 /6550 Directory

TDSL Personal Edition

The Reference 6J5 Thread (L63, 6C5, 12J5, 6P5, etc.)

The Reference Pentode and Beam Tetrode Thread EL34, KT66, KT77, KT88, EL11, EL12, EL3N, EL35, EL37, 6550, 6L6, EL51, EL39, 4654, 4699, KT63, KT61, 6V6, etc

Russian Vacuum tubes equivalence table / Таблица аналогов ламп

Educational videos:

 

[OctavianH]

Procedure to determine the Grid Bias Voltage for Output stages​

Before you start:

Be sure to check post 1) and 2) and understand the tube rolling principle.

Be aware that wrong operation might damage your amplifier.
DO IT AT YOUR OWN RISK.
Nobody here takes any responsibility for any damage.​

Introduction:

The builder will provide operating points of each amplifier to the owner. He will also provide values for most common tube types in a table and those should be used if they are available. However, several users tried to present a way to calculate these values and I tried to summarize and present them here. The owners of the informations and credits are found below on the thread.

Needed information for calculation:

- operating points of the amplifier for each stage (Vplate / Iplate)

Method 1 (recommended) - Use the Tube Datasheets and Triode connection curves

Steps:

1. Find the appropriate datasheet of the tube you want to use
2. Search inside for the Triode Connection curves as a graph of Vplate (V) vs Iplate (mA)
3. Carefully check the operating points of your stage (might differ on each build, please check with the builder) as a pair of Vplate (V) and Iplate (mA) values
4. Intersect on the graph the Vplate (= your operating stage voltate) with Iplate (= your operating stage current) and obtain a point on the graph
5. Estimate based on the graph the Vgrid (several fixed values are presented as an exponential function).

Example 1: 6V6 as Triode in the Output Stage.

1. The datasheet of a 6V6 is this one:

Hint: A good place to search for datasheets is The Valve Museum and the fastest way to find the tube you need is a simple Google Search:

For 6V6 we have this one. At page 4 we have the Triode Connection graph.
If we consider our operating points of the output stage as Vplate = 200V and Iplate = 20mA the intersection of these values on the graph is close to Vgrid = -15V:

Hint: Always try to round to a bigger negative value (but not more than 1V), ensuring a lower current than the one you want to set and later adjust it. In this way you are sure not to damage your amplifier internal circuitry.

Method 2 - Use the calculator

There is an output stage calculator available here.

Steps:

1. Choose Operation Mode = Triode
2. Choose Vplate (= your operating stage voltate)
3. Choose Iplate (= your operating stage current)
4. Calculate the Grid Bias Voltage

Example 1: 6V6 as Triode in the Output Stage.

Applying the settings presented above for the same operating point described in the previous example we obtain the following value:

The recommended value using the calculator is smaller than the one from the tube graphs (-13.29V vs 14.5V) and was most probably determined by measured values of existing tubes.

Builder's remarks on these 2 methods and why graphs are better is here.

Builder's example of grid calculation for Infinity here.

Tube Compatibility Guide​

• TBD

Tube rolling combos
(impressions, pictures, links)​

• TBD

Clarification needed:

• TBD

 

[m-i-c-k-e-y]

Thank you for starting this . Now I can browse thru at peace at the other thread .

 

[SonicTrance]

@OctavianH
Great posts!

3. Stop the amplifier wait 5 min and insert the tubes in the sockets

Fixed it for you! That's to let the power supply caps drain before inserting the tubes.

 

[OctavianH]

Fixed it for you! That's to let the power supply caps drain before inserting the tubes.

Updated. I have just tried to gather the information in the other thread and structure it. It is just the begninning but I think it is already enough for someone to have an overview and to be able to understand what you are building with this new add-on.

 

[SonicTrance]

Note: You cannot use all 3 inputs (single and double triodes) at the same time, however all tubes will glow because the one not selected as input will not have the plate and grid connected.

This is only the case if you get an input tube selector switch. If no switch you can only have either the 6J5's or the 6SN7 socket populated at the same time.

 

[OctavianH]

This is only the case if you get an input tube selector switch. If no switch you can only have either the 6J5's or the 6SN7 socket populated at the same time.

Ok, I will update it. The only 2 builds I know of are having 3 inputs but you are right, other builds will come which might be different.

 

[SonicTrance]

Ok, I will update it. The only 2 builds I know of are having 3 inputs but you are right, other builds will come which might be different.

There might have been some misunderstanding. Levi's Infinity will not have the input selector like yours. So he can't have all input sockets populated at the same time.

 

[OctavianH]

There might have been some misunderstanding. Levi's Infinity will not have the input selector like yours. So he can't have all input sockets populated at the same time.

Ok, I removed that part completely as it creates confusion. I removed also the question related to tube wear when not selected. These remain specific parts which we remove.

 

[OctavianH]

I started to be very curious about EL39 today, after what I have read on the original thread here. I do not own any EL39 in my collection or adapters for it but as far as I know it is the predecessor of EL34. Now, EL34 has a relaxed sound and great bass, which makes me think about KT77. I have a pair of KT77 and now I wonder how close are these to EL39. Is anyone here who tried on the same amplifier EL34/KT77 and EL39 and can tell use if those were close?
I searched for a clear datasheet and could not find about these.

 

[hpamdr]

I started to be very curious about EL39 today, after what I have read on the original thread here. I do not own any EL39 in my collection or adapters for it but as far as I know it is the predecessor of EL34. Now, EL34 has a relaxed sound and great bass, which makes me think about KT77. I have a pair of KT77 and now I wonder how close are these to EL39. Is anyone here who tried on the same amplifier EL45/KT77 and EL39 and can tell use if those were close?
I searched for a clear datasheet and could not find about these.

The EL39 was designed as an Audio tube for amplifier in movie theater being powerfull and reliable the RFT have special construction with anode on ceramic and gold plated grids The grids are not aligned and this make the tube in pentode mode a bit less efficient than EL34. Once strapped as triode, you get the magic
The closest datasheet you can find are for the 4654 / EL50 tubes. [I got the info from Jean Luc Benoist, he built some version of U.F.O amp with it and still use the tube in the new COVID amplifier... ]

 

[OctavianH]

I want to try them but I think to postpone this until I will try what I have. This means EL32 and EL38. I have to admit that I prefer to use "native" tubes instead of ones with adapters. For me EL32 is a very special tube, very good on vocals, piano or orchestral music. The problem comes when electric guitar comes into play and since I listen 99% to rock music I guess you realize how often that happens. Until now, my all time favourite for guitars is KT66, but with Eternity I am not sure if KT66 will sound the same as I know it. So the question remains if what I love at KT66 is the proper biased sound or maybe the distorted one. We will see more about this in the future.

In short terms:
- I do not like tubes with top cap, however I like a lot how 6F8G or EL32 sounds
- I do not like the EL3N base and hate the adapters for it (a few broke already)
- I do not like also the EL11 base, even if it is much better than EL3N

In my ideal tube world all tubes are octals, work natively, are cheap and sound perfect. OK, time to wake up now.

Later edit:
I think this adapter works on Eternity/Infinity for EL39 as output.

 

[hpamdr]

I want to try them but I think to postpone this until I
In short terms:
- I do not like tubes with top cap, however I like a lot how 6F8G or EL32 sounds
- I do not like the EL3N base and hate the adapters for it (a few broke already)
- I do not like also the EL11 base, even if it is much better than EL3N

In my ideal tube world all tubes are octals, work natively, are cheap and sound perfect. OK, time to wake up now.

Later edit:
I think this adapter works on Eternity/Infinity for EL39 as output.

Yes this adapter will work, another solution can be to have removable anode cap on the amp to use el39, el36...
About adapter, it would be better if you can build them or ask to a better builder than ebay.

 

[OctavianH]

Yes this adapter will work, another solution can be to have removable anode cap on the amp to use el39, el36...
About adapter, it would be better if you can build them or ask to a better builder than ebay.

I never took too seriously to build my own adapters because I rarely need to make a new one and to buy spare parts for 2-3 adapters never made sense for me. Another reason is that I have 2 left hands and I am not sure if the time and effort makes sense. However I plan to try to make myself some interconnects, because the prices exploded in that area, so maybe I will also try to make myself some adapters. If not there are several sources where you can obtain better adapters than Ebay.

Regarding EL39 the information is not very good, but I found a Philips 4654K datasheet:

And from this one I deduced the pinout:

Now, the datasheet does not have any triode connection curves so it is not very clear for me what grid bias needs.