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Hey guys!

I haven't necessarily given up on winding transformers, although with school the way it is right now, and me not being able to live without a tube amp, I figured I'd go ahead and buy the transformers. They WILL get wound this summer, however!

I got my transformers today and put them in. I have biased the amp to 45mA per tube. I figure that's pretty reasonable. I've yet to build a preamp as I thought I could enjoy it with my iPod only. I hooked it up, and it's very quiet. It sounds good, from what I can tell, but it's way too quiet to enjoy.

I read in the manual that 1.3V RMS will drive the amp to 35W. However, I am using the Blackburn Audio 6SN7 driver board. I haven't a clue what it needs to drive the EL-34s to 35W. My EL-34s are matched by transconductance, but only one channel has "matched" tubes. The two matched ones are made by Ampex.

I then tried hooking my iPod to my Onkyo 5.1 surround sound receiver. Then I took the headphone out to the input of the ST-70. It got it much louder, but there was some awful clipping. It definitely wasn't putting out 35W. I am thinking that the Onkyo could be clipping, but then again, I only had it at 0dB (halfway on the volume control).

By the way, I am using two Dynaco A-25 speakers (8 ohm).

Any help/insight would be greatly appreciated!

Kyle

P.S. Pictures are soon to come!!

Hi Kyle ...

Is this amp a "new rebuild" ?

I'd suggest if you just built the driver board, or if it's otherwise
new and untested, check all of the resistor values, to make
sure someone didn't make the common mistake of using
a 100K where a 1K goes, etc.

You should be able to drive the amp to a listenable volume
with an iPod.

Double check the wiring.

... tom

antiquekid3 wrote:I got my transformers today and put them in. I have biased the amp to 45mA per tube. I figure that's pretty reasonable. I've yet to build a preamp as I thought I could enjoy it with my iPod only. I hooked it up, and it's very quiet. It sounds good, from what I can tell, but it's way too quiet to enjoy.

I do not have an iPod, so the following should be taken with a grain of salt (or a case of gunpowder).

http://members.chello.nl/~m.heijligers/ ... ments.html

Are you using the iPod's headphone output jack? If so, there could be an impedance mismatch between the iPod and the amp. The headphone's output impedance is only 5 Ohm. Its output voltage is supposed to be 2.91V.

http://beavishifi.com/articles/headphonejack/

The line out output impedance is 100Ohm (a bit low), but from what I can tell, you need an Apple Universal Dock to access it. Its output is 2V, so it will drive your st-70 to ear-shattering levels - directly - without a preamp.

http://store.apple.com/us/product/MB125

Alright...as if the mystery couldn't get stranger!

First of all, I think at 17 (almost 18) I should be able to tell the difference between "yellow-purple-yellow" and "yellow-purple-black"!!! Haha...! Anyways, there were two resistors that went from the grid of one half of a 6SN7 to the plate of the other. So I had a 47 ohm in there instead of a 470K. No wonder it was so muffled, I guess!

However, I have since replaced the resistors (and double checked...and triple checked...), but now, the cursed amplifier is oscillating!! Every time it's gone into oscillation, I've quickly turned it off (hopefully I haven't blown anything!!). I swapped around the tubes and the noise didn't really follow around; it didn't matter what EL34s or 6SN7s were in there. I've changed all of the 6SN7s with different ones and it made no difference.

So...where should I go from here?! :-(

Kyle

Ah ! Don't feel bad, everyone has done it, hence it's one
of my first troubleshooting suggestions.

Your oscillation is probably feedback because your transformer
primaries need to be reversed. Another common problem.

The negative feedback is positive ... just reverse the transformer
ends, including screen taps and I think all will be well.

Tom, aren't you a genius?! To give myself a little credit (only a wee bit, now!) I did switch them around before I read your post. And now I'm listening to Dark Side of the Moon!! And it sounds great! I'm addicted already! Too bad I have to go do some homework now...but I have a feeling I'll be enjoying it a lot tomorrow!

Thanks so much for all of your help!! I couldn't have done it without y'all!

Kyle

P.S. Pictures are coming soon! I promise!

Glad that everything came out in the wash.

Back on the subject of a preamp. If you decide to add a turntable to your system, modern magnetic phono cartridges will need a preamp ahead of the ST70 to give full volume. (But, you'd need a preamp anyway for the RIAA equalization.)

I like the ST-70 with a MooseFET or BoZ preamp anyway - gives some extra control with crummy (usually A/V) sources.

Cheers!

Hey Geek,
I'm not too bright about the things you just mentioned!! Could you please give an example of each type? Thanks, Vince

BoZ = the Nelson Pass "Bride of Zen":
http://cygnus.ipal.org/mirror/www.passl ... nbride.htm

MooseFET, a derivative of the BoZ for simpler power supplies and a very triode-like FFT spectra:
http://geek.scorpiorising.ca/moosefet.html
(PCB's coming)

Cheers!

Yeah, I'm so glad it's finally working, and working very well might I add! I have noticed the bias voltage drops by almost 100mA from three minutes after turn-on to maybe 2 hours afterward. Is this pretty normal? I've since biased them to 40mA per tube. I'm using matched Amperex EL34s from Great Britain (not Ampex, like I said in an earlier post) for the left channel and an RCA and an Amperex "Bugle Boy" for the right channel. Now that I know this works, I might invest in a matched quad. We'll see!

I checked out the BoZ and MooseFet. I was thinking about originally making a tube preamp, but I assume the MOSFET design would be more "transparent." That'd be fine with me.

How's this for a phono preamp? http://members.myactv.net/~je205d/varEQph-1.jpg

I'll start a new thread with pictures soon.

Kyle

antiquekid3 wrote:I checked out the BoZ and MooseFet. I was thinking about originally making a tube preamp, but I assume the MOSFET design would be more "transparent." That'd be fine with me.

MooseFET fooled some people at her debut that she was a tube ;-)

How's this for a phono preamp? http://members.myactv.net/~je205d/varEQph-1.jpg

Never tried it, so I can't say myself.

Cheers!

The phase oscillation is a classic tube amp problem. You never forget it! Congrats!

I must be confused here. What is the purpose of a preamp - unless it's for a phono input, with an RIAA equaliztion circuit? Line-level sources shouldn't need a preamp.

The preamp links above don't seem to have RIAA equalization.

I'm sorry, did I misread? I thought the original question was on straight line pre's 0_o

A preamp is very handy even with line level signals as they give a low driving resistance and therefore are make the signal less sensitive to cable variance.

In the case of an ST-70, it needs a volume control anyway.

Preamps can also add with their various non-linearities, a personal quality (if you want pure transparancy, ditch the tube amp and go SS ;-)

Cheers!