Ok, since Donald is on board I'll shoot a technical question: I plan on using one of the lot of PCC88/7DJ8 NOS tubes I got. Reading the specs, I noticed that the heater works at 7V instead of 6.3V (not much of a difference according to many), but more importantly that it's designed to be connected in series instead of in parallel like the ECC88/6DJ8.

From what I remember from the YAHA thread I read when I built mine, this should mean that the LM317 should be configured for fixed current instead of fixed voltage. Am I right?

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they should work the same ECC88 PCC88 are both connected to pins 4/5 with pin 9 connected to a screen. You may be thinking of some of the 12V tubes where the heater supply can be connected between 4/5 and 9 to run the filaments in parallel and allow 6.3V operation.

If you wanted to adjust the voltage to 7V then exchange one of the resistors for a trimmer or use sockets to tweak the heater voltage. ( the trimmer will probably not tolerate such high current )

..dB

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In the YAHA thread on headwize fa-smith made a point a few times of making sure to configure the LM317 for fixed current iwth series heated tubes, and fixed voltage for parallel heated ones. To my limited knowledge it makes sense, and the schematics for the PCC88 report that it's wired in series. Pins are the same, yes.

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I will probably do that as I'd like to try both the PCC88 and the ECC86 which I used in the YAHA.

I'm assuming that R110, a 1K trimpot, is for adjusting the bias on the output stage. Is this correct? And, if so, is there any reason it couldn't be replaced with a fixed, say, 330R resistor?

I ask because I'm playing around with a PCB layout for this amp, in the hopes of producing a design that's a little more user- and noob-friendly than the existing one. (Not that the 8audio one is bad, but there are a few things I think could be done better.)

(At the same time, I hit upon an interesting idea, but I'm not familiar enough with tubes to know if it's workable: This amp design uses a dual triode, yes? What would need to be changed to make the design a mono amp - both triodes in the tube amplifying a single signal? Is it just as simple as tying the output of one section to the input of the other? I'm thinking it could be amusing to have fully-independent channels, one tube per channel, while still keeping things simple and low-voltage. Tiny-monoblock-tube-headamp, anyone?)

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I asked a simliar question to Donald by email a couple of days ago, here is his quick reply.

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Interesting idea on the mini monoblocks.

I am planning to use ECC86 for this amp once my PCB arrives, as they low voltage tubes and might work very well here. And I have a local source of Philips NOS ones, not super cheap but decent (10 euros), I already used one in my YAHA. The MEHA used an ECC86 with 24V too, and there's a detailed explanation on how to calculate the cathode resistor, if anyone is interested.

Edit: I love mini hybrid amps like this one, the YAHA or the SS. They are an easy build and perfect to learn some tubes theory and experiment with it.

For now, I'm just sticking with the regular, double-channel amp. While I've managed to get everything in a space around 2.5 by 3.5 inches - which is not a whole lot larger than a Pimeta - I'm ambivalent whether there's any real benefit to having a smaller, somewhat more flexible design (pot and jacks offboard, meant to be panel-mounted; some of the TO-220 devices on the board edge, to allow more, larger heatsink options, et cetera).

In the meantime, I've now got most of the parts to build the amp, so should be able to start playing around with this by next weekend.

Does anyone have a BOM for this amp? I'm still getting up to speed amp building and it's tough to figure out what the parts list is and where to get all of them. For example, the schematic calls for a diode and I have no idea what part # that would be. I would also like to get a Hammond case (or similar) to fit this PCB, but can't exactly figure out how to size these (1455N1602BK, perhaps?).

I don't think there's a real BOM, as such. Part of the fun is the flexibility.

The diode is a LED, which lights when the amp is turned on. It's designed so that you can stick it under the tube, if you like. I don't like, so I'm just using a 5mm green LED, but if you want to go that route, you'll probably want to use a 3mm high-intensity LED.

You'll need a 6DJ8 or equivalent tube, and a nine-pin (sometimes called "noval"), PCB-mount socket. I'm using something like this; you can also go for one of these.

There were a number of links for the Japanese transistors earlier in this thread; BG101 is a TO-220 LM317, like this one.

You can pick up the potentiometer, and a DC power jack that will work just fine, from Amb.

For resistors, I'm using generic 1/4W 1% metal film ones, from a $12 assortment on eBay; it's unclear whether R101/R201 really need to be 1W resistors, as specified in the schematic. If so, this should work quite well. R103/R203 and R107/R207 seem to be the only resistors "in the audio path", and you could use "boutique" resistors here if you really wanted, like Mouser 660-SPR2CT521R100J for R107/R207.

The capacitor choices probably aren't too critical; use whatever decent caps you can find for most of the positions (Panasonic FM, or whatever); C101/C201 and C102/C202 are the input and output signal coupling caps, respectively. I'd suggest some kind of nicer axial-leaded cap for the '01 position; I'll probably use some Soviet PIO caps, or maybe some smallish ERO poly caps, here. I have my doubts about the '02 cap positions; I doubt 1000uf is really necessary, and will be trying some 3.3uf Wima film caps instead.

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20 minutes after I posted my question it occurred to me that the diode is an LED. Duh.

Thanks for the helpful parts links, I was having trouble finding a direction on the right parts, especially the tube sockets. I will probably build this after I'm done with my Millett SS.

R104/R105 (and R204/R205 ofc) will dissipate quite a lot of power as there's the heater current going through them. I will use 1W resistors (uhm, maybe I got 2W can't remember right now).

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Yeppo, thats right.

Depending on your mood, you can bypass the cathode resistor with a cap which may have put the original design beyond budget Some would argue that the CCS on the plate of the tube minimizes the need for a cathode bypass cap, but its cheap to test.
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It controls the operating point of the tube. adjusting it will give different sounds, and accommodate variances in different tubes.

The use of a CC(sink) to set the idle current of the output mosfet AND run the heater is pretty cool. Davistik posted this up as an idea for a high voltage alternative to the starving student.

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So the heaters on these tubes don't make them glow very much huh... I'll have to stick an orange LED under mine i guess lol

Most miniature tubes don't really glow all that much, if operating correctly. (I had a leaky 50EH5 in a radio once which glowed a fairly bright blue, but it wasn't supposed to!) Heck, most of the older octal tubes didn't glow all that much, either.

So, yes, if you want your tube to really glow, you'll need to light it with an LED. (I've always thought that for added fun you could turn the LED(s) into a very crude VU-meter... maybe use an RGB LED? Hmmn...)

That could be cool but with a tube amp an analog vu meter would look more at home. In my opinion anyways.

Right now I'm working on building the power supply for this thing. I have the transformer I just need to build the regulator circuit for it which I have mostly done in Eagle. This will just be a basic 2 amp 24v regulator to get used to the transformers. I'm really interested in breaking out the center tapped 36v transformers!

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I don't think you want to put 3.3uF for C102 and C202. Read one of the replies to a similar question in the Starving Student thread. 470uF are probably fine, which is what most people use in the Starving Student. And maybe R107/R207 could be dropped.