Just bought a Dirty Shirley clone with an advertised 22w power rating for a good price on ebay and upon receiving it opened it up and found it had a Hammond 1608A output transformer, this transformer has wide bandwidth 30hz - 30khz and is rated for 10watts. The person I bought it from is adamant it has at least 20w as the hammond is under specced , I am finding it very hard to believe it can put out 22w with a 300v a side power transformer.
Could he possibly be right? Or am I right and deserve my money back
Output transformers
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Re: Output transformers
My understanding is that a core that's good for 10W at 30Hz should be good for 20W at 60Hz, 40W at 120Hz etc.
Obviously the windings overheating will become a limitation at some stage.
I guess your amp is a copy of a DS mini, using a push pull pair of EL84?
I've read posts from builders successfully using 10W OTs from Hammond's hifi line in such amps.
If you're happy for the EL84 to be pushed hard, then a real world 20W or more may be feasible, eg 30W theoretical max
Vg1-k=0 is the uppermost plot
Obviously the windings overheating will become a limitation at some stage.
I guess your amp is a copy of a DS mini, using a push pull pair of EL84?
I've read posts from builders successfully using 10W OTs from Hammond's hifi line in such amps.
If you're happy for the EL84 to be pushed hard, then a real world 20W or more may be feasible, eg 30W theoretical max
Vg1-k=0 is the uppermost plot
Last edited by pdf64 on Tue Mar 05, 2024 11:07 am, edited 2 times in total.
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Re: Output transformers
Sorry forgot to mention it's got a pair of 6v6s
Re: Output transformers
No problem, the HT may need to be a bit stiffer, eg bigger reservoir cap.
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Re: Output transformers
I am working on a real DS right now trying to take some gain out of it, but more importantly hiss.
Anyway with 100% strong testing 84s in this amp and 120 vac coming in the amp in stock form is hard pressed to output 12 watts rms.
That Hammond will live forever in that circuit if the core is close to the same mass as the original OT.
Anyway with 100% strong testing 84s in this amp and 120 vac coming in the amp in stock form is hard pressed to output 12 watts rms.
That Hammond will live forever in that circuit if the core is close to the same mass as the original OT.
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Output transformers
Probably the best way is to put it on the oscilloscope and find out, if I had one 
Luckily I can borrow one tonight and put it to the test.
Luckily I can borrow one tonight and put it to the test.Re: Output transformers
pdf64 is right about transformer power handling. It scales up and down with the lowest frequency it has to handle, until wire overheating comes into play. 10W down to 30z really does mean 20W at 60Hz and so on.
Re: Output transformers
I was going to use a 1khz sine wave is this the best or use different frequencies For an accurate representation ?
Re: Output transformers
what is the DC voltage on the amp?strat100 wrote: ↑Tue Mar 05, 2024 6:33 am Just bought a Dirty Shirley clone with an advertised 22w power rating for a good price on ebay and upon receiving it opened it up and found it had a Hammond 1608A output transformer, this transformer has wide bandwidth 30hz - 30khz and is rated for 10watts. The person I bought it from is adamant it has at least 20w as the hammond is under specced , I am finding it very hard to believe it can put out 22w with a 300v a side power transformer.
Could he possibly be right? Or am I right and deserve my money back
with 300VDC it is not possible to have 22W.. also with 400VDC and 8kAA transformer.. not possible.. 1-2W are only losses in OPT.. so on the speaker will be 12-16W.. OPT is not a problem, it is exactly in optimum
Re: Output transformers
Plate voltage is 405dc and biased at 65% 22.6ma.
Tested on the oscilloscope and getting around 16 Watts, it's not 22 watts as advertised but is it enough to warrant sending it back
Tested on the oscilloscope and getting around 16 Watts, it's not 22 watts as advertised but is it enough to warrant sending it back