I bought these amplifiers from an old man at work.
He brought them in because he heard i love tubes and he wanted to be rid of the heavy turds at 80 something years old...
I offered him $500 for these and the preamp which was also a diy effort.
The workmanship was poor and the amps didn't work. The boards were a rats nest and the builder used 5 different types of green wire.
I bought them for half of what I figured the iron was worth because I knew i could do something with them eventually.
(The preamp has SUTs)
I drew out the schematic and it did not make sense and it also didn't work, so I knew I'd need to overhaul the whole thing in order to make something worthwhile.
Luckily the A-340 dynaco is an extremely easy to use transformer. I could copy the dynaco mk2/3 but I already had 2x small tube sockets.
Instead I went full Williamson.
I am in fact William's son...
I put 6cg7 in the front sockets for va/pi
And driver.
I built a fr4 circuit board with a bias/balance adjustment pot.
I rewired everything using cloth wrapped wire to Retma color codes.
Now they both fire up and bias in and it's time to put signal through them and start fine tuning them.
He brought them in because he heard i love tubes and he wanted to be rid of the heavy turds at 80 something years old...
I offered him $500 for these and the preamp which was also a diy effort.
The workmanship was poor and the amps didn't work. The boards were a rats nest and the builder used 5 different types of green wire.
I bought them for half of what I figured the iron was worth because I knew i could do something with them eventually.
(The preamp has SUTs)
I drew out the schematic and it did not make sense and it also didn't work, so I knew I'd need to overhaul the whole thing in order to make something worthwhile.
Luckily the A-340 dynaco is an extremely easy to use transformer. I could copy the dynaco mk2/3 but I already had 2x small tube sockets.
Instead I went full Williamson.
I am in fact William's son...
I put 6cg7 in the front sockets for va/pi
And driver.
I built a fr4 circuit board with a bias/balance adjustment pot.
I rewired everything using cloth wrapped wire to Retma color codes.
Now they both fire up and bias in and it's time to put signal through them and start fine tuning them.
Those things do look like a mess downstairs... Hope the changes you've made work out for you.
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Three photos...one in the dark...two under the amplifier...please post a photo from above the amp.t Possible an heathkit amplifier-circuit .
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They were a mess downstairs but I cleaned them up nicely.
I used a Williamson schematic i found in radio and television news.
It's written by Herb Keroes.
I got the power tubes biased around 70 miliamps each and ran it up to clipping and got a whopping 22 watts.
After an hour or so the power transformer is hot.
At full power the B+ has sagged by about 100v from 420v to a little over 300.
The original amplifiers use way too much output transformer and not nearly enough power transformer so I'm stuck between a rock and a hard place.
Eventually I'll put smaller output transformers on the old girl and it'll be a nice 25 watt amplifier.
I used a Williamson schematic i found in radio and television news.
It's written by Herb Keroes.
I got the power tubes biased around 70 miliamps each and ran it up to clipping and got a whopping 22 watts.
After an hour or so the power transformer is hot.
At full power the B+ has sagged by about 100v from 420v to a little over 300.
The original amplifiers use way too much output transformer and not nearly enough power transformer so I'm stuck between a rock and a hard place.
Eventually I'll put smaller output transformers on the old girl and it'll be a nice 25 watt amplifier.
.
What is the current of el34 .. in the no signal Mode?
Midnightmayhem.......I got the power tubes biased around 70 miliamps each and ran it up to clipping and got a whopping 22 watts.
What is the current of el34 .. in the no signal Mode?
Roughly 70 miliamps each, as described in the mullard data sheet for distributed loading with 430v on the plates.
At rest B+ is ~430v
At full power just before clipping b+ is ~300v
What does that tell you?
It tells me that the original builder was frugal, greedy, or both.
He wanted more from his cheap power transformer than it would deliver, but was unwilling to go ahead and buy 2x higher capacity units....
Too bad, so sad, it's a 25 watt amplifier with 50 watt output transformers that are good to well over 100w.
Something has got to go...
At rest B+ is ~430v
At full power just before clipping b+ is ~300v
What does that tell you?
It tells me that the original builder was frugal, greedy, or both.
He wanted more from his cheap power transformer than it would deliver, but was unwilling to go ahead and buy 2x higher capacity units....
Too bad, so sad, it's a 25 watt amplifier with 50 watt output transformers that are good to well over 100w.
Something has got to go...
An Antek toroidal power transformer is affordable, and there are many rated for the power this amp wants/needs to put out.
How do you test the maximum signal? By injecting a 1 kHz signal?Please send a photo of the top of the chassis.For curiosity.
I test the maximum signal by injecting a sine wave into the grid of the voltage amplifier using my Hewlet Packard oscillator.
After correcting a few errors (I had the bias way off due to using .5ohm cathode resistor instead of my normal 1.0 ohm.)
I got it to crank out 32 watts. Into my 8 ohm dummy load just before clipping and it clips gracefully after that.
Next I'm going to try and dial in a little feedback and once that's done I'll listen to them for a bit. My square wave generator bit the dust and I don't have the transformers I'm going to eventually use so final feedback tuning will have to wait, I'm just hoping for something listenable....
After correcting a few errors (I had the bias way off due to using .5ohm cathode resistor instead of my normal 1.0 ohm.)
I got it to crank out 32 watts. Into my 8 ohm dummy load just before clipping and it clips gracefully after that.
Next I'm going to try and dial in a little feedback and once that's done I'll listen to them for a bit. My square wave generator bit the dust and I don't have the transformers I'm going to eventually use so final feedback tuning will have to wait, I'm just hoping for something listenable....
That's a nice amplifier. I built the Acrosound version with a pair of TO-330s. Don't bias the output tubes too hot, you won't get enough grid excursion for full power. The EL34 grids should read about -38 volts. You shouldn't have any trouble getting 45-50 watts out of it with a proper power tranny. The circuit is tricky because the RC coupling arrangement to the output tubes applies positive voltage to the EL34 grids, which then must be countered by a larger-than-normal negative bias supply. If the bias supply fails, the grids will go positive and the tubes will melt down before you can grab for the power switch. ;-) And I'd suggest that you won't really "hear" that amplifier until you apply the required 20dB feedback. You'll definitely need a low-pass filter at the input to tame some ringing. But those Dynacos are easier to work with than the Acros, which tend to have excessive high-frequency energy.
I hadn't.
Just put a 22k in that position, swapped the grid leads and measured it by adding the feedback and the output dropped 15db
Just put a 22k in that position, swapped the grid leads and measured it by adding the feedback and the output dropped 15db
Curious, are you really using a 5V4 rectifier? A 5AR4 would be even better, but yours looks like a 5U4 which has a higher voltage drop giving you more sag. Or a 5R4 that's even worse.
So, I tidied up my choice if bias resistor and my choice of feedback resistor and with 22db of feedback I measured right at 35 watts at the onset of clipping.
On a frequency sweep I got -3db points of 10hz and 60khz.
I settled on a 18k feedback resistor and it calculates out to about 22db of global negative feedback.
Bias holds steady and even so I decided to go ahead and put them into my system for a listen.
They don't suck.
I think i prefer the 6l6 to the EL34 but that's really splitting hairs...
These sound good!
On a frequency sweep I got -3db points of 10hz and 60khz.
I settled on a 18k feedback resistor and it calculates out to about 22db of global negative feedback.
Bias holds steady and even so I decided to go ahead and put them into my system for a listen.
They don't suck.
I think i prefer the 6l6 to the EL34 but that's really splitting hairs...
These sound good!
Listened all night last night.
The tubes on the right hand amplifier have some red glow going on so I need to dial back the bias current on them, overall they sound great. Nice tight bass and a good even presentation top to bottom. They were worth the entry fee and all the work...
I went to bed at 11 last night and my landmate shut them off when he finally crashed, at who knows what hour of the night!
The tubes on the right hand amplifier have some red glow going on so I need to dial back the bias current on them, overall they sound great. Nice tight bass and a good even presentation top to bottom. They were worth the entry fee and all the work...
I went to bed at 11 last night and my landmate shut them off when he finally crashed, at who knows what hour of the night!
