General questions before finishing up ST-35 clone

[ThSpeakerDude88]

Hi! Thanks to a lot of good advice from you guys, my amp is almost finished. All I have left is to order a few caps and resistors ( as well as a full tube compliment).

The chassis is finished , and heater, power, primary and output wiring is done.

Here is a picture so far, I will post more of the wiring soon.
http://getchellaudio.googlepages.com/IMG_0348.JPG

So far, here are my questions.

1: What brand of tubes would you reccomend for both the 12dw7 and the 6bq5s?
2: ( sort of same as first) I am seriously considering the Sovtek EL84M, if they are any good. Also , Mullard has some new production EL84s.

3: How much power supply capacitance? Original had 60/40/20, I was thinking about 100/100/47(?)

4: Reccomendations for good quality /sounding potentiometer for volume? I was just going to throw a cheap $3 one in there, but I already have a decent sum of money invested.. I'll spend up to $10 on a pot if it makes a diff.

5: Resistors. Carbon comp,film, or metal film?

6: I already have a thread on capacitors, but I think I am going with auricaps. What are the critical ones that really affect the sound? IE, do I need to do just the coupling caps, or also the ones in the FB loop?

7: Feedback loop. I am using Hammond 1650F's ( why , I don't know, I think its because I alrady had one, but I will probably buy z565's later on.) Since the 1600 series transformers have weird output hookups instead of decending taps, do I just increase resistance in the FB resistor and tie it to the 8 ohm tap, or does the extra 8 ohms of resistance matter that much?

8: cathode biasing. I am thinking about putting a 100 ohm pot between the cathodes on each channel, and then a seperate 100 ohm resistor on the wiper to ground. This would allow me to fine tune the cathodes and balance them out. Might be easier with two seperate pots, but that takes up more space.

9: Heater ref- ok as it is,off of the cathodes?

I don't know if I have forgotten anything or not, so please feel free to critique and tell me your opinions.

Here are a few pics to enjoy-
http://getchellaudio.googlepages.com/IMG_0348.JPG
http://getchellaudio.googlepages.com/IMG_0322.JPG
http://getchellaudio.googlepages.com/IMG_0349.JPG

[ThSpeakerDude88]

*bump*

Also forgot to mention another question... The input filter. Can I just get rid of the input resistor (47k) and hf filter capactior?

[ThSpeakerDude88]

anyone?

[SY]

1. For 6BQ5/EL84, I've gotten good results from JJ.

3. I'd go lower on the first cap- high C there means high ripple currents and more strain on the rectifiers- and higher on the last C.

4. You won't do better than the cheap Alps pot sold at Radio Shack until you're willing to pony up something more than $10. The RS pot is actually pretty good.

5. Depends on position. Ideally, matched wirewound for plate loads and both the loads for the inverter, carbon comp for grid stoppers and power supply, metal film for grid and cathode resistors (except the split load cathode).

6. Coupling caps are the most critical. I'd use Wima FKP or MKP before I'd spend money on "audiophile" caps.

7. If you take feedback from the 8R tap, you need to decrease the feedback resistor by 30% or so. You'll also want to carefully adjust compensation, which you'd need to do anyway given the new output transformer.

8. Don't bother. It's useful to split the cathodes between channels, but with matched JJ output tubes, a single cathode resistor per channel is as complicated as you need to get.

9. Bias us the heater string to +60V or so, bypassed to ground.

[Ty_Bower]

If you haven't already, you really need to go spend a few days reading all the threads here:
http://www.diytube.com/phpBB2/viewforum.php?f=1

I think someone has already answered just about every question you could ever hope to ask concerning the ST35 clone.

1: I've had good luck with the Russian 6pi14pi tubes (Reflektor). Especially considering the price, I wouldn't hesitate to recommend them.

2: If the Sovteks are anything like the Reflektors, they should hold up well.

3: Look at the power supply schematic on the DIYtube ST35 site.

4: I'll second the Radio Shack volume control. It works fine for me.

5: Dunno. I have little experience with "boutique" parts.

6: Ditto on the caps.

7: http://www.diytube.com/phpBB2/viewtopic.php?t=60

8: Balancing the bias current between each side of the output pair is a good thing. Mismatched bias current results in standing DC in the output transformer, leading to early core saturation, reduced max output and greater distortion. The DIYtube circuit uses four separate trimmer pots, one for each output tube. If your tubes are matched, that's great. If they're not, a trim pot makes it real easy to fix.

9: Not sure I followed the question...

[Ty_Bower]

Originally posted by ThSpeakerDude88
Also forgot to mention another question... The input filter. Can I just get rid of the input resistor (47k) and hf filter capactior?

I'll assume you are referencing R1 (47k) and C1 (.1 mfd) on the Dynaco schematic. If you are certain your signal source has no DC, then you should be able to eliminate C1. I'd probably still keep R1, mostly because I'm not entirely sure what it does (grid stopper?).

[ThSpeakerDude88]

hey thanks for the replies, I just signed up for diytube but they havent activated my account yet.

Funny you mention the RS pot, I have one I was thinking about using. It IS an alps pot, so it should be pretty decent. I think as long as my pre-source has balance control I should be fine.

Thanks for the link on the feedback , I think I got it from there now.

I have looked up the DIY st35 schematic for the power supply, and a few other mods. What value would you reccomend for the first capacitor? I noticed the DIYst35 uses two 150uf caps in paralell ( creating a 300uf low ESR cap). I already have two new 100uf 450v illinois caps, and a 47uf for the PI filter.

I read a good article not too long ago on resistors. They basically said the same thing you said, metal films in the cathodes, ww for the plate load , and carbon comps for signal path.

As for tubes, Mullard, Tung-Sol, EH and SOVTEK are all made in the reflektor plant. I have read that the SOVTEK and EH's are rough copies of the old mullard designs.

TY, The capacitor I was referring to is the one that goes to ground from the input leg. I can't remember which one this is, but I have read it was there to supress RF. If I don't have a problem with RFI, I would assume I can just leave it out to open up the amp a little bit?

[Ty_Bower]

Originally posted by ThSpeakerDude88
I have looked up the DIY st35 schematic for the power supply, and a few other mods. What value would you reccomend for the first capacitor? I noticed the DIYst35 uses two 150uf caps in paralell ( creating a 300uf low ESR cap). I already have two new 100uf 450v illinois caps, and a 47uf for the PI filter.

TY, The capacitor I was referring to is the one that goes to ground from the input leg. I can't remember which one this is, but I have read it was there to supress RF. If I don't have a problem with RFI, I would assume I can just leave it out to open up the amp a little bit?

I'm not sure which DIY ST-35 schematic you are reading. I've got a "Rev B" which uses a pair of 220uF, 250V caps in series to make an effective 110uF, 500 volt cap up front. Later revs used a single 120uF, 450 volt rated part. Either should be fine if you've got solid state rectifiers, per the original Dynaco design. Some DIY ST35ers have gone with a tube rectifier. In that case, I'd keep the first cap under 40uF or so. In any case, 300uF sounds like overkill to me. I doubt you need that much capacitance anywhere in the PSU.

That input cap (C2 on Dynaco's schematic) is for RF rejection. Here's a link to a discussion thread:
http://www.diytube.com/phpBB2/viewtopic.php?t=1707

[ThSpeakerDude88]

hey thanks , I checked out that link earlier actually. Seems like I'll be doing it without it. I am however going to keep the input capacitors, to protect the amp from any DC ( in the case of my weird computer/ soundcard problems I have)

I'm just doing the standard SS rectification, so extra capacitance won't strain it. I do have a GZ34 laying around somewhere...hmm I might have to try it sometime