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Rebuilding an old tube amp.

post #1 of 25
Thread Starter 

Dear Forum,

 

   About a year ago I pulled this little guy from my grandparents old sound system:

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AppleMark

I constructed the cage and installed the power switch and light in the front. Now, a year has passed and I now have some free time to work on it. Presently It does work, however, it has the typical hiss associated with components this age, also the tone controls and gain either no longer function or function so poorly they are unusable. 

 

This brings me to the main subject of this topic. I would like to strip out all the basic components from this amplifier leaving really only the transformers, and rebuild it as a tube headphone amplifier. I was looking at the possibility of basing a design already implemented on an amazing amplifier like Woo Audio Wa6, but don't know if there is a better design out there that might not leave me with the task of drawing out a schematic for the Wa6 based off only two pictures I found online of the underside of the unit. I am still somewhat new to the world of audiophile based amplifiers, which is why I bring my question to you guys. If presented with this situation, what would you do? Thanks for any help in advanced, I would like to take this back to college this fall and use it in my listening setup.


Edited by wroachbarrette - 7/25/13 at 7:49am
post #2 of 25

Generally with stuff that's from the 50s and 60s, the electrolytics need replacement, sockets need to be cleaned, carbon comps could have potentially drifted, and the film caps should be checked. Parts in old tube amps can sometimes be difficult to find replacements for, such as strange valued potentiometers, wirewound resistors and multisection caps, although you can always simply use a multitude of new smaller caps and stuff them in the chassis. On the bright side, it's a relatively simple circuit and should be easy to work on. On the negative side, the likelihood that it will work well with headphones isn't all that high. The OPTs in the unit likely have outputs for 4/8 ohms (maybe 16) and most headphones are 32+, so you'd have to consider switching them out. Most speaker amps, when adapted for headphones, can be a bit noisy.

 

My suggestion would be to start from the ground up with an amplifier designed for headphones, something like the Bottlehead Crack, the Millett Starving Student or MiniMax, or the Beezar ECP Torpedo. Otherwise I think you may find yourself disappointed. As always, be sure to review basic safety precautions (some are listed here) as tube amps are often high voltage.

post #3 of 25
Thread Starter 
Quote:
Originally Posted by HiGHFLYiN9 View Post

Generally with stuff that's from the 50s and 60s, the electrolytics need replacement, sockets need to be cleaned, carbon comps could have potentially drifted, and the film caps should be checked. Parts in old tube amps can sometimes be difficult to find replacements for, such as strange valued potentiometers, wirewound resistors and multisection caps, although you can always simply use a multitude of new smaller caps and stuff them in the chassis. On the bright side, it's a relatively simple circuit and should be easy to work on. On the negative side, the likelihood that it will work well with headphones isn't all that high. The OPTs in the unit likely have outputs for 4/8 ohms (maybe 16) and most headphones are 32+, so you'd have to consider switching them out. Most speaker amps, when adapted for headphones, can be a bit noisy.

 

My suggestion would be to start from the ground up with an amplifier designed for headphones, something like the Bottlehead Crack, the Millett Starving Student or MiniMax, or the Beezar ECP Torpedo. Otherwise I think you may find yourself disappointed. As always, be sure to review basic safety precautions (some are listed here) as tube amps are often high voltage.

I've heard of the Bottlehead Crack but need to research the rest. As for the precautions, yes I am well versed and have even had my fair share of mishaps with tubes in particular. "Those electrolytics caps can really shock you :P" thanks for the input. Which one of these tube amp designs do you find to be your personal favorite "if you have one that is?"

post #4 of 25

I am sure there are many here that can comment on the sound reproduction quality. But as far as build quality/components used /circuit concentrate on the Bottlehead Crack and Beezer Torpedo . The bottle head uses the well know -6080- which has usually high reliability- The   circuit of "cathode follower" ensures a low output impedance and high drive power from the tube and is direct coupled which is good for detail -no capacitors in series with the output.   The Beezer uses very high quality components and for the price you get a lot including custom transformers. I would pick this one right away if it wasn't for the output capacitors which must affect the reproduction and frequency response. --I visited a website with comments by an audio engineer. I agree with his comments in the "toss up " between the 2 . So although the Beezer is high quality. I would go for the  Crack   due to the cathode follower configuration  as it is compatible with a wide range of headphones-- I REALLY do like the DIRECT connection to the headphones.   

post #5 of 25
Quote:
Originally Posted by duncan1 View Post

I am sure there are many here that can comment on the sound reproduction quality. But as far as build quality/components used /circuit concentrate on the Bottlehead Crack and Beezer Torpedo . The bottle head uses the well know -6080- which has usually high reliability- The   circuit of "cathode follower" ensures a low output impedance and high drive power from the tube and is direct coupled which is good for detail -no capacitors in series with the output.   The Beezer uses very high quality components and for the price you get a lot including custom transformers. I would pick this one right away if it wasn't for the output capacitors which must affect the reproduction and frequency response. --I visited a website with comments by an audio engineer. I agree with his comments in the "toss up " between the 2 . So although the Beezer is high quality. I would go for the  Crack   due to the cathode follower configuration  as it is compatible with a wide range of headphones-- I REALLY do like the DIRECT connection to the headphones.   

I'm sorry, but this is totally wrong.

 

The Bottlehead Crack is a great amp, but it is a classic OTL (Output Transformer-Less) design.  It most definitely uses large electrolytic capacitors in series with the output.  In fact, that's practically the definition of a "cathode follower."  It is not suitable generally for low-impedance headphones because the output capacitors form an RC circuit with the headphones that results in a bass filter.  Bottlehead recommends headphones with a minimum impedance of 100 ohms right in their manual: http://www.bottlehead.com/et/samplemanuals/Crackmanualsample.pdf (p.6 under "What the Crack kit is about.")

 

The Torpedo, on the other hand, is a transformer output amplifier that is well-suited for either high-impedance or low impedance phones.  It does extremely well with Grados and Sennheisers, either one.  The circuit topology is a "parafeed" design, where the cathode is grounded and AC-coupled to the output transformer.  However, in the connection between the tube output and the output transformer, the impedances are unlike an amp that has capacitors on the output (see paragraph above).  Very small values of capacitance may be used, allowing the use of very high-quality film capacitors (only 4.7uf in the Torpedo).  There is nothing on the output connection itself except a direct connection from the output transformer(s) to the headphone jack.   


Edited by tomb - 7/27/13 at 11:58am
post #6 of 25

Sorry I was looking at a modification on another audio website  elsewhere. Where   the circuit was was modded to remove the output cap. I did say the Beezer was the better quality than the  Crack the only dispute was the output capacitor.So yes you are right the standard Crack has an output cap.

post #7 of 25

Sorry, not meaning to hijack this thread, but I'd be interested in an output cap-free crack mod. A cursory 5 minute google search didn't reveal any such page. Any chance you could post a link Duncan?

 

on topic: +1 for the Beezar amps, If you were happy with a hybrid amp, then I bet you could get a Millett Mosfet Max or MiniMax or minimax into that enclosure. Tack something like the woven raffia material that they use on guitar cabs over the hole on the front to end up with a nice looking and good sounding amp without breaking the bank. If you want fully tubes then something along the lines of the old earmax (with the Cavalli-Jones updated schematic) is a nice place to start for a simple decent sounding compact circuit


Edited by Goobley - 7/27/13 at 2:13pm
post #8 of 25

The problem is that they were taken from another website  although the "author" did not admit it and I don't want an argument elsewhere.But you should click on--www.tubecad.com>brains+brawn cathode follower.. If you scroll down far enough you will see some which were used to modify the design--do not click "next" at bottom right but click on blogxxx 1 +blogxxx 2 at bottom of the writing for more .Sorry I cant be more helpful but I don't want repercussions on this elsewhere.

post #9 of 25
Quote:
Originally Posted by duncan1 View Post

The problem is that they were taken from another website  although the "author" did not admit it and I don't want an argument elsewhere.But you should click on--www.tubecad.com>brains+brawn cathode follower.. If you scroll down far enough you will see some which were used to modify the design--do not click "next" at bottom right but click on blogxxx 1 +blogxxx 2 at bottom of the writing for more .Sorry I cant be more helpful but I don't want repercussions on this elsewhere.

I don't think you're going to suffer any repercussions.wink.gif  If you're referring to me, I was only correcting an incorrect post (granted, it was maybe a firm correction).  Hell, I make mistakes all the time and people post right afterward, "You're wrong!!".  Or, in words similar to that anyway ... redface.gif

post #10 of 25
Thread Starter 

Thanks for the responses, right now I am using the ATH A700 "around 64 ohms" and the ATH M50'S "about 38 ohms,"  And if possible, I was wondering if I could ask about the issues that accompany using the OPTs I have now even if they are only rated at about 4-16 ohms. I honestly was just looking at transforming this amplifier while spending the least amount of money possible, its really more of a fun project than one that I expect will leave me with something that's equivalent to a new expensive tube amp. I understand that the OPTs can become noisy, but is that the only drawback?

post #11 of 25
OPTs won't become noisy, tubes will, but that's true in any circuit. HiGHFLYiN9 is correct that you likely need to replace the capacitors, and possibly the resistors. This will probably eliminate a bunch of your noise issues. Some of it may be power supply noise, in which case you'll need a better PS to eliminate it. You can find everything you need at places like tubesandmore.com. However, be sure you understand the safety implications of working on an old tube amp. I'd look at the tube forum at diyaudio.com for help.

As for the existing OPTs, they may well not be that good in modern high fidelity terms, but for a fun project they should be fine. Because of the high Z on the phones, you should probably put a resistor in parallel to provide the proper load. For instance, on an 8 ohm tap, place a 9 to 10 ohm resistor in parallel with either of your phones for a decent load. Use one that has a several watt rating to keep it from being damaged.
post #12 of 25

I believe for better help you should mention what you have.  It's a single ended 6BQ5 (EL84) amp from an old Magnavox console.  Around 4W.  The two 6BQ5's are driven by a super hi mu 6EU7 dual triode.  Rectifier is 6CA4.

It's a really nice amp.  I have a few recommendations:

Remove the bass/treble circuitry - install a better volume pot and go right to the input tube grid (might need grid stopper resistor but pretty sure I did not use one).

Remove the feedback circuit.

The tubes run on AC heat - even with indirect heaters I've never run them on AC with headphones.  Go DC for less noise but will be super difficult to fit in that little box.

Not much filtering in the PS - might need more for dead quietness.

Even with the feedback, the gain of the 6EU7 is still too high - with feedback removed, I have not replaced my 6EU7 but the intension is to replace with a lower mu and hopefully better driver tube.

Those OPT's are good! and please don't think twice about plugging in 32ohm phones to the 8 ohm sec.  The impedance will reflect back to the sec and result with higher sec impedance (4x) - no issues for what you are doing/experimenting.  

But remember there is no harm in getting yourselt a headphone jack and solder from the OPT secondary to the jacks - listen and decide if you want to move forward.  (start with volume at zero - remember you have 4W there and the very high gain).  But remember there are huge sonic and noise improvements to be made.  Also check out other single ended 6BQ5 headphone amp designs for comparisons sake.

If you don't like it, please contact me and I will be glad to take it off your hands :-) OR get/make yourself a pair of high efficiency single driver speakers, plug in line in device of your choice, and sing and dance - perfect kick ass college room setup.

 

Edit:

I don't remember exactly but you have around 300VDC B+ with power on - don't touch without shutting off and verifying caps are drained / B+ = 0.


Edited by macm75 - 8/2/13 at 5:09am
post #13 of 25
Thread Starter 
Thanks for the reply! I didn't know that much about the amplifier and its really appreciate that you informed me. I have already soldered a headphone jack onto the amp and I replaced all the caps. I managed to cut down on some of the noise just by soldering some resisters onto the output circut just before the jack "crude I know but it did something" thanks for the bit about switching to DC . I don't know much about that to be honest and will do some research to figure out what I'd like to do with that. Right now though I have to say it does sound really nice though and I deciding if I want to take it a step further and cut out the tone controls and replace the gain pot. I'll keep you posted. Again thanks for the info
post #14 of 25

Good work on cap replacement - the signal caps between the plate of each 6EU7 triode and the grid of each 6BQ5 (get yourself the data sheets to see what pins I mean) would be good to swap in particular.

Regarding DC, there are a million ways to skin this - at least for temporary experimentation.  Right now there is one winding on the power transformer that has the ~6.3VAC supply for the heater of all 4 tubes - wired in series to each one.  If you wanted to see what DC heat does, take the wires off the first tube from PT heater supply and externally extend the wires to a bridge rectifier, ~10KuF 'lytic cap,  ceramic power resistor (to drop to 6.3VDC give or take 5% - wattage calculated by total volt drop times total current times 3-5 for safety), and another ~10KuF cap - this back to the first tube heater pins.  Or if you find an external linear or switched mode ~6V PS (with enough current supply for all 4 tubes - again check data sheets) just remove heater wires at the first tube (from PT) and wire to new PS (of course black tape the ends of the 6.3VAC wires from the PT).  Sometimes with SMPS's they are still noisey - follow them up with 'lytic filter cap and dropping resistor if required.  Don't turn on the amp - just run the heater PS, see glow, and measure to see if all tubes get the required heater voltage - if so, then power on amp and listen.

Golden rule is one change at a time - for instance don't touch B+ PS filtering until you finished listening to results of heater PS change.

I wonder if there is a schematic out there for your amp - do a search for Magnavox 6bq5 singled ended.  I remember way back seeing someone who went off the charts with improvements - including CCS loads on driver tube anodes.

post #15 of 25

I would pick this one right away if it wasn't for the output capacitors which must affect the reproduction and frequency response.

8.gif

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