Super Rocky
- Cressy Snr
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#91 Re: Super Rocky
Thursday last, I pushed the “one thing well” philosophy to its logical conclusion and reduced the series cathode resistor combo on the output stage to one component, giving a value of 270R, down from 420R. This puts the KT77 op point at 66.5mA and -19V bias, giving 21.7W anode dissipation, with the HT I have from a GZ37 fat bottle.
I could probably, screw a little bit more power out of it with a GZ34, but I don’t want to push these modern production valves too far, so that’s the output stage rectifier rolling and general p*ssing about pathway shut down too. Anyway, as it stands, I think I’ve probably reached the best compromise between power output, distortion and valve life using the voltages and currents provided by a GZ37.
It also means the the amp only works with KT77s as EL34s over-dissipate with those cathode resistors and that HT. There is little room for manoeuvre, valve wise now, as there are only two current makers of the KT77: JJ and the New Sensor Corporation, under the Gold Lion brand name. I might try a couple of JJs at some point. It’d be churlish not to.
The KT77 also works properly with 2K5 output transformers, which is not the case with EL34s or the 6L6 types, so making the thing for KT77s only, ought to have been a no-brainer, and that was the original intention, but you know what I’m like when I can’t leave well enough alone. Anyway, deliberately using 270R cathode resistors has shut off firmly, any more messing with the power stage, as has using a 4V UU5 on the input stage PSU. The rectifiers are sewn up, the output valve type is set. The only real opportunity for tube rolling now is with the 6SL7 input valves.
I’ve put the schematics below along with the brands/types of resistors and caps used in the PSUs and the audio circuit:
Coupling cap in the audio circuit is a Mallory 150 series polyester jobbie. Cheap arse crap with no place in a hi-fi amp? Maybe, but they give the fat, vintage tone and clean treble I want from the system for the music I play.
I could probably, screw a little bit more power out of it with a GZ34, but I don’t want to push these modern production valves too far, so that’s the output stage rectifier rolling and general p*ssing about pathway shut down too. Anyway, as it stands, I think I’ve probably reached the best compromise between power output, distortion and valve life using the voltages and currents provided by a GZ37.
It also means the the amp only works with KT77s as EL34s over-dissipate with those cathode resistors and that HT. There is little room for manoeuvre, valve wise now, as there are only two current makers of the KT77: JJ and the New Sensor Corporation, under the Gold Lion brand name. I might try a couple of JJs at some point. It’d be churlish not to.
The KT77 also works properly with 2K5 output transformers, which is not the case with EL34s or the 6L6 types, so making the thing for KT77s only, ought to have been a no-brainer, and that was the original intention, but you know what I’m like when I can’t leave well enough alone. Anyway, deliberately using 270R cathode resistors has shut off firmly, any more messing with the power stage, as has using a 4V UU5 on the input stage PSU. The rectifiers are sewn up, the output valve type is set. The only real opportunity for tube rolling now is with the 6SL7 input valves.
I’ve put the schematics below along with the brands/types of resistors and caps used in the PSUs and the audio circuit:
Coupling cap in the audio circuit is a Mallory 150 series polyester jobbie. Cheap arse crap with no place in a hi-fi amp? Maybe, but they give the fat, vintage tone and clean treble I want from the system for the music I play.
Last edited by Cressy Snr on Sat May 16, 2020 7:47 pm, edited 2 times in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
- Cressy Snr
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#92 Re: Super Rocky
Anybody want a NOS pair of MO Valve Gold Lion KT77?
https://www.ebay.co.uk/itm/NOS-GENALEX- ... Swl9BWLrVo
Cheap at twice the price I say,
https://www.ebay.co.uk/itm/NOS-GENALEX- ... Swl9BWLrVo
Cheap at twice the price I say,
Sgt. Baker started talkin’ with a Bullhorn in his hand.
- Mike H
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#93 Re: Super Rocky
Yup. Love the drawings.I’ve put the schematics below along with the brands/types of resistors and caps used in the PSUs and the audio circuit:
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
- Cressy Snr
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#94 Re: Super Rocky
I always like to put up the schematics.
Although this circuit is obviously nothing to write home about, it does work well and sounds good. It’s probably the twin power supplies that raise the sound of this circuit from “ho hum” to “not bad at all”
These amps are good value for money and give decent sound into simple, efficient speakers with benign electrical loads.
I mean lets be honest; these are not the same quality level as well executed DHT amps. They are however, easy to work with, fun to build and glow nicely.
Although this circuit is obviously nothing to write home about, it does work well and sounds good. It’s probably the twin power supplies that raise the sound of this circuit from “ho hum” to “not bad at all”
These amps are good value for money and give decent sound into simple, efficient speakers with benign electrical loads.
I mean lets be honest; these are not the same quality level as well executed DHT amps. They are however, easy to work with, fun to build and glow nicely.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
- Cressy Snr
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#95 Re: Super Rocky
It’s dead
Power stage mains transformer has just caught fire.
Guess that’s the end of the valve journey again. I’ve had enough.
Power stage mains transformer has just caught fire.
Guess that’s the end of the valve journey again. I’ve had enough.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
- izzy wizzy
- Old Hand
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#96 Re: Super Rocky
Bummer! But at least you know you have a good sounding amp. All it needs is a replacement mains tx and you're good to go.
- Cressy Snr
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#97 Re: Super Rocky
I’ve got another mains TX and its already painted blue. First job’ll be to find out why the hell it burned. HT secondary is fused solid. TX is still too hot to touch.
NVA clone is back for now. At least that’s something. The TX transplant can wait until tomorrow. Not a difficult job but bloody fiddly as there will have to be a lot of dismantling to get at the wires. A few days work.
NVA clone is back for now. At least that’s something. The TX transplant can wait until tomorrow. Not a difficult job but bloody fiddly as there will have to be a lot of dismantling to get at the wires. A few days work.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
- IslandPink
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#98 Re: Super Rocky
"Far from ideal" as Harry Hill would say !
"Once you find out ... the Circumstances ; then you can go out"
- Cressy Snr
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#99 Re: Super Rocky
Well I suppose it’s something to keep me occupied and out from under her feet.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
- izzy wizzy
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#100 Re: Super Rocky
Good you have another. Not trying to tell you how to fault find .... are the HT caps high enough V rated? With choke input, it can rise to V peak before settling to it's final value depending on how things get going on first turn on.
- Paul Barker
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#101 Re: Super Rocky
Agree, check power supply for resistance to ground.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
- Cressy Snr
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#102 Re: Super Rocky
All that was taken care of. Shunt resistors draw the critical current at all times.izzy wizzy wrote: ↑Wed May 27, 2020 8:04 pm Good you have another. Not trying to tell you how to fault find .... are the HT caps high enough V rated? With choke input, it can rise to V peak before settling to it's final value depending on how things get going on first turn on.
Resistance to ground is infinite, the solid state diodes in the hybrid bridge are both fine.
The output stage supply is now somewhat academic as the blasted ‘replacement‘ transformer is way too high voltage. I’d thought the thing was identical, but it isn’t going to work.
So... I have a 200VA toroidal isolation transformer with 2 x 115V secondaries. This means that the output stage is going to have to be cap input, to get 325V before rectifier drop. What that’ll do to the sound quality doesn’t bear thinking about.
Fiddlesticks!
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#103 Re: Super Rocky
What spec transformer do you need Steve? Maybe one of us has something suitable lurking on a shelf somewhere?
Sorry, I couldn't resist!
#104 Re: Super Rocky
I've got a few transformers steve, I have some around 350 0 350 to 370 volts, it's the ma rating I'm not sure about.. They are all old ones though
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
- Cressy Snr
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#105 Re: Super Rocky
Panic over chaps.
I’ve found something that’ll work. Voltages are the same as the burned out one, but the VA rating is double. It’s an old Danbury one I bought years ago. It fits where the other one was, but the mounting holes will have to be re-drilled, as will the cable entries in the chassis. It needs testing first, but it worked last time I used it.
Onwards! (albeit slowly and carefully)
I’ve found something that’ll work. Voltages are the same as the burned out one, but the VA rating is double. It’s an old Danbury one I bought years ago. It fits where the other one was, but the mounting holes will have to be re-drilled, as will the cable entries in the chassis. It needs testing first, but it worked last time I used it.
Onwards! (albeit slowly and carefully)
Sgt. Baker started talkin’ with a Bullhorn in his hand.