Supro inspired amp to build soon just finished 1st schematic

[swingarm]

Hey thanks for the info. I usually don't buy transformers this way. I wish I could though . I buy old donor amps and transformer lots when they present themselves. I recently scored an abandoned London power "standard" amp project as described in tut 5 from Arkansas on Ebay. All the transformers chassis boards tube sockets and most of the switches and a few pots. It was one of my best scores ever! $56 shekels and about 45 shekels to ship. Here are a few pics from the ebay auction. Hope you don't mind me gloating about it The only problem is that many high voltage areas were wired with 1/4 watt resistors...oh well.













Here is a link to a guy that built one, if you are interested.

http://www.fydamps.com/photo.html

OK now back to the thread.

The amp I'm using for this project is an old Davis 106 (el84 power tubes) I seemed like a good amp to experiment with since I already have it. I've used one before in an 18 watter with good results. The purists will be charging blasphemy since I'm not using 6973's I would like to use those but just cant justify the cost, they are priced way to high for me. The new production EH (i think) offering is reported to be a different tube with a different sound.No first hand info on that.

[swingarm]

OK so here is the almost complete layout its on terminal strips. Chuck I decided to go with my original bias scheme. I appreciate your help. Have a look and find some errors please folks. The wiring in the amp will not look like this as the tone post are very close to the tubes. same with the inputs. Not sure if I did those inputs right. layout supro for the davis 106 amp terminal strips.pdf I realize it might be difficult to decipher, the buzz fix is in the layout as is the original proposed bias circuit. I'm wondering if there is a problem with the bias. There is a 270k resistor on the terminal strip between v3 and v3 that is grounded coming off the 33nf coupling cap. Just seems this could be a problem for the bias circuit no? (Chuck: This is where you say why didn't you just use my suggestion concerning the bias) LOL. I just want to be able to bias each tube independently and so can use new and old tubes together. Does that make sense? The bias schem below gives "some idea" of whats going on there

[Chuck H]

I've got to be honest. No one likes to follow layouts or read text descriptions of circuits for circuit evaluation. Please submit circuits for evaluation in schematic form. After that a layout could be evaluated for things like ground scheme and possible stability issues due to component proximity. Schematics submitted for evaluation should be complete for the circuit in question. To be fair, we've all done it. That's how we know it's a thread killer. I can see you put some time into the image already. That's good. Try to see the other side though and imagine evaluating someone elses circuit with a layout and text description instead of a schematic

[swingarm]

Thanks Chuck. I know what your saying. In fact a layout with terminal strips is even worse The sound of crickets made that painfully obvious. I went and had a good cry (you know the kind...I brought my knees right up into my chest and sobbed myself into exhaustion After I awoke I fired up the soldering iron and started. I'm about 1/3 done. Kinda foolish, your right, I should post a schematic. Maybe today sometime.

[Chuck H]

I went and had a good cry (you know the kind...I brought my knees right up into my chest and sobbed myself into exhaustion

I do it in the shower and think of pancakes. Clothing optional.

[swingarm]

OK here is the power amp schematic.
supro power amp davis 106.pdf

Edit: put 470k resistors in parallel with diodes to correct the schem

[swingarm]

Here is a shot of what I've done so far

[Leo_Gnardo]

Here is a shot of what I've done so far

Where did that front shot of your Chucky-amp go? VERY amusing! At first I was aghast that a Davis amp was going to be gutted - got some very respectable relatives by that name & I've been hoping to get my hands on some of those oldie-but-goodies - but it looks like you've done well. And we're all getting along now so much better too. Koom-bay-yah everybody, hands across the water and entente cordiale, all that stuff.

[swingarm]

For you Leo my man.

[swingarm]

OK done for the day, I'm missing a few parts -zeeners for the buzz fix and 1 ohm 1 watt resistors. but here is the progress.

getting there



Koom-bay-yah everybody, hands across the water and entente cordiale, all that stuff. Indeed however I must say Germany was never treated properly but I suppose that's water under the bridge I guess

[Chuck H]

I don't have a program for modification of PDF files or to save them as alternative files. I need to ask what your attachment is to the original bias switching circuit.?. I actually see some (possibly?) unwanted peripheral to it's operation. The load for each side of the PI signal is different due to parallel resistors, the injection point for the actual bias voltage (k?) is either directly grounded or looped across a 10M resistor and the 25k pots (individual adjustments?) really won't offer much range. I'm not even sure what a lot of this arrangement is supposed to achieve. A switched bias, even with individual adjustment, should be a simpler circuit. Unless you're married to it for reasons unknown I think you should consider an already tried and proven circuit. There are a few amps that use do this. The Fender Prosonic and a couple of Mesa amps come to mind. Then, of course, there's my ultra simple diode on a switch method Of course that doesn't have the added affect of additional plate to cathode voltage that the other methods do so the effects would be less dramatic. On that note...

I had an amp for awhile that used a pair of El84's in fixed bias. I added a cathode bias switch to that amp. Very low drama. Even trying to be a cork sniffer tone snob the difference was virtually insignificant. Part of the reason is that EL84's use a very small bias voltage. They do exhibit considerable bias shift in cathode bias but it really didn't wow you to hear the difference. So there's that. Let me put it this way. I always encourage experimentation, but... I've done it. And if I had it to do again I probably wouldn't bother.

[swingarm]

I had an amp for awhile that used a pair of El84's in fixed bias. I added a cathode bias switch to that amp. Very low drama. Even trying to be a cork sniffer tone snob the difference was virtually insignificant.

I actually see some (possibly?) unwanted peripheral to it's operation. The load for each side of the PI signal is different due to parallel resistors, the injection point for the actual bias voltage (k?) is either directly grounded or looped across a 10M resistor and the 25k pots (individual adjustments?) really won't offer much range. I'm not even sure what a lot of this arrangement is supposed to achieve. A switched bias, even with individual adjustment, should be a simpler circuit. Unless you're married to it for reasons unknown I think you should consider an already tried and proven circuit. There are a few amps that use do this.


Yes individual bias.

Really not much difference? sigh...oh well. In the larger amps I have built the benefit was the ability to use any octal in the same family but in this case it seems the only real benefit is the ability to use new tubes and old tubes together. I think you are right , ...if there is no real difference between cathode and fixed bias sound in el84 amps. However as you can see I'm committed to the bias scheme now. I guess we will know soon, that is if there are no major issues. I have used this bias scheme successfully in 4 other amps. Its employed in all the amp projects in the TUT series.

BTW I use DIYLC to make my drawings in case you were interested.

Thanks Chuck.

[swingarm]

Just wondering, maybe if I didn't use the bypass cap in cathode bias I might get some tonal variation?

[Chuck H]

Just wondering, maybe if I didn't use the bypass cap in cathode bias I might get some tonal variation?

A few guys here have tried that and reported on it (not me though). I've read that it adds more noteworthy sag and difference. If the sag gets too, uh, saggy... You could probably add low value resistors to a bypass cap to dial the effect in to taste.

I see you're committed now. In case you find the need to alter anything this is how "I" might implement a switchable cathode/-V grid bias w/ PR mod, measurement resistors and individual bias control. No disrespect to KOC but I think it's an eminently simpler and more eloquent circuit.

[Dave H]

Just wondering, maybe if I didn't use the bypass cap in cathode bias I might get some tonal variation?

I don't know what it sounds like but the waveform is horrible when the bypass capacitor is omitted on a class A/B amp.