The 12B4 ought to do well if it's selected.
The curves for g2 current on the plate voltage plots do not look particularly linear... remember, these are biased close to zero volts at idle and are driven past 100V. There's a pretty nice change in Ig2 through that swing, I'll wager. In any case, the horrid little bits are IRF820.
The curves for g2 current on the plate voltage plots do not look particularly linear... remember, these are biased close to zero volts at idle and are driven past 100V. There's a pretty nice change in Ig2 through that swing, I'll wager. In any case, the horrid little bits are IRF820.
I have a good stock of 12B4A, so I think I´ll give them a try.
So, I can see a vague image of what this project is going to look like when finished:
4 x 807
2 x 12B4A
2 x 5AR4
Various preamp and phase splitter tubes (6SL7 and 6SN7 probably)
1 x Hammond 1650T OPT
1 x BIG mains transformer
1 x smaller mains transformer
1 x BIG PSU choke
1 x smaller PSU choke
1 x coupling transformer
various big lytics and PiO´s
This one will be a BEAST!
I don't like IST's, they have wild responses. It's bad enough having one transformer in the loop, let alone two. Plus there's no control past the driver's plate (low Z as a 12B4's is).
I'd go for a high Gm cathode follower. 7KY6's are cheap...
A MOSFET is just a really SFA high Gm tube, but it has massive capacitance and a tendancy towards being siliconD) so doesn't really interface well with tubes.
Tim
I'd go for a high Gm cathode follower. 7KY6's are cheap...
A MOSFET is just a really SFA high Gm tube, but it has massive capacitance and a tendancy towards being silicon
D) so doesn't really interface well with tubes.Tim
Hi there......In the 1960's these 807 tubes were used en masse in guitar amps...as they were cheap and plentiful RF surplus. They worked well at high B+ when other tubes had restrictions, also the high value paper smoothing caps in psu's were the norm. Although I did a scathing report on the KT90..... thinking about it I wished Mullard did a 50W version of the EL34; then perhaps we would have got real claim for fame.
A pair of 807's strung at 600V P-P fixed bias and screen at 300V and 6K5ohms a-a will sport 80Watts in AB2 at 3.5% THD.
The capabilities of this tube shouldn't be underestimated and the emphasis of a slam driver stange is just as important.
rich
Nanana: I use the old fashioned way; paper and pencil.
Don´t have a clue what other people use though.
Sch3mat1c: I´m a bit uncomfortable with cathode followers beacuse of the following reasons:
They require a negative PSU rail and I suspect there might be some startup issues before everything is warmed up and all the voltages has settled.
Not sure about the last part, but I remember that I had some problems some time ago when I was messing around with 811A´s and cathode followers.
I´m not too worried about the IST, the transformer seems to be of reasonable quality and since this will be a bass guitar amp I only need a response from 35-40Hz up to maybe 8kHz or so.
Don´t have a clue what other people use though.
Sch3mat1c: I´m a bit uncomfortable with cathode followers beacuse of the following reasons:
They require a negative PSU rail and I suspect there might be some startup issues before everything is warmed up and all the voltages has settled.
Not sure about the last part, but I remember that I had some problems some time ago when I was messing around with 811A´s and cathode followers.
I´m not too worried about the IST, the transformer seems to be of reasonable quality and since this will be a bass guitar amp I only need a response from 35-40Hz up to maybe 8kHz or so.
Sch3mat1c said:
A MOSFET is just a really SFA high Gm tube, but it has massive capacitance and a tendancy towards being silicon
Massive capacitance, yes, but not only that - the capacitance itself is non-linear
Tendancy towards being silicon... LOL
richwalters said:
I share the sentiment, Rich, but ssshhh! You'll be driving the price up
(Ok, maybe not for a while, there are so many 807s and similar valves....)nanana said:
Yep, of course we do, jc. But they tend to be screenshots or scanned handwritten ones. Not everyone has the ability to read DXF files and the like.
Fuling said:
Well, you'll need a negative rail anyway for bias. You can just run the biaset voltages to the CF grids and return the cathode/output grids to the same rail. It'll have to be a little stiffer to handle the bias current, but a 0A3 or 0B2 can fix it in place so you don't have to worry about drift.
Using high Gm drivers, you don't have to worry about drift, or even much voltage shift (which you'll fix anyway by adjusting the bias voltage based on current..). I'd expect only a few volts drift, negligible when you're halfway cut off, near class B. That's the nice thing about cathode followers.
Tim
hey all,
merry christmas!
i have a schematic here of a class AB2 845 pp amp i designed that blackie built into an even older 845 class A pp amp i built about 10 years ago. the problem was that the owner changed speakers and started using planar ribbons. he needed alot more power than the 30 watts he got from the class A pp 845s. the maximum supply voltage was only 1000 volts so the only way to really increase the power was to go AB2 or B. to improve the efficiency, in other words. so the driver stage and power supply became a little more complex...! but this amp made 166 watts into 4 ohms @ 5% thd with the same supply as the 30 watt class A amp. that tells you why class AB amps are the commercial norm and not class A. i will find a picture of the amps and post it too...
i had a time converting the autocad file into a GIF (thanks SY!) but it works sort of... if i could have economically increased the main B+ to 1200 -1300 volts, this beast would have easily hit 200 watts. there are only two coupling caps and the inductors are fender twin output trannies without using the secondary and a custom plate choke wound for me by jack eliano. the second owner damaged one of these so he replaced them both with yet another fender replacement trannie, a bassman i think? the output trannie is 5K 200W from jack. i think i remember the -2dB point as being 40 something khz at 150W. anyway, just as an excercise to see if i could do schematics here...
jc
merry christmas!
i have a schematic here of a class AB2 845 pp amp i designed that blackie built into an even older 845 class A pp amp i built about 10 years ago. the problem was that the owner changed speakers and started using planar ribbons. he needed alot more power than the 30 watts he got from the class A pp 845s. the maximum supply voltage was only 1000 volts so the only way to really increase the power was to go AB2 or B. to improve the efficiency, in other words. so the driver stage and power supply became a little more complex...! but this amp made 166 watts into 4 ohms @ 5% thd with the same supply as the 30 watt class A amp. that tells you why class AB amps are the commercial norm and not class A. i will find a picture of the amps and post it too...
i had a time converting the autocad file into a GIF (thanks SY!) but it works sort of... if i could have economically increased the main B+ to 1200 -1300 volts, this beast would have easily hit 200 watts. there are only two coupling caps and the inductors are fender twin output trannies without using the secondary and a custom plate choke wound for me by jack eliano. the second owner damaged one of these so he replaced them both with yet another fender replacement trannie, a bassman i think? the output trannie is 5K 200W from jack. i think i remember the -2dB point as being 40 something khz at 150W. anyway, just as an excercise to see if i could do schematics here...
jc
Attachments
Question for nanana
nanana,
I would like to try a simple AB2 mod to a Fender Super Reverb. I would like to use a 12au7 to drive the grids of the EL34s into conductance. I'm trying to eliminate the collapse of the LTP PI 12AT7 when the grid goes positive. I'm not looking for more power just more of the good kind of distortion (harmonic) and less of the bad kind (crossover)
I have looked at the SVT as a starting point. It looks like I'll have to find a way to pickup a 150V B- supply somehow. My questions are:
Is there an easy way to do this?
How do I trade off adjustable output tube bias with a low impedance B- supply that will sink the grid current?
Will I have to regulate the output screen supply and run the screens at a lower voltage?
Is there a schematic out there for this kind of mod?
Thanks. I'm a real newbie to AB2 but Iwant to try it.
nanana,
I would like to try a simple AB2 mod to a Fender Super Reverb. I would like to use a 12au7 to drive the grids of the EL34s into conductance. I'm trying to eliminate the collapse of the LTP PI 12AT7 when the grid goes positive. I'm not looking for more power just more of the good kind of distortion (harmonic) and less of the bad kind (crossover)
I have looked at the SVT as a starting point. It looks like I'll have to find a way to pickup a 150V B- supply somehow. My questions are:
Is there an easy way to do this?
How do I trade off adjustable output tube bias with a low impedance B- supply that will sink the grid current?
Will I have to regulate the output screen supply and run the screens at a lower voltage?
Is there a schematic out there for this kind of mod?
Thanks. I'm a real newbie to AB2 but Iwant to try it.
Another Question for nanana
nanana,
I had another idea. Instead of using direct coupled cathode followers, what about using a phase splitting driver transformer. I could replace the 12AT7 with a cathode biased triode connected EL84.
I'm sure there is something wrong with this idea. It sounds too easy to do. If it will work, could you recommend a driver transformer?
nanana,
I had another idea. Instead of using direct coupled cathode followers, what about using a phase splitting driver transformer. I could replace the 12AT7 with a cathode biased triode connected EL84.
I'm sure there is something wrong with this idea. It sounds too easy to do. If it will work, could you recommend a driver transformer?
Re: Another Question for nanana
www.sowter.co.uk has a selection. I haven't used this technique for decades. The emphasis is on a precisely made transformer with equal winding capacitances....this has to be compromised with other factors....study the sowter spec and insist on more info regarding balances etc......
If unequal winding capacitances are present then considerable thd occurs at upper audio freq's due to phase differences. Also the leakage inductance is important for bandwidth and distortion, all these best left to a skilled transformer manu.
Sonically worth a try ?? Perhaps others have tried it out
rich
www.sowter.co.uk has a selection. I haven't used this technique for decades. The emphasis is on a precisely made transformer with equal winding capacitances....this has to be compromised with other factors....study the sowter spec and insist on more info regarding balances etc......
If unequal winding capacitances are present then considerable thd occurs at upper audio freq's due to phase differences. Also the leakage inductance is important for bandwidth and distortion, all these best left to a skilled transformer manu.
Sonically worth a try ?? Perhaps others have tried it out
rich
Thanks Rich.
Those Sowters look like a little more than I need. A guitar has a bandwidth of about 80Hz to about 5 to 6 KHz. and a little bid of harmonic distortion is not a terrible thing.
I was looking at the Hammond iron because it's cheap.
http://www.hammondmfg.com/124.htm
What do you think?
Those Sowters look like a little more than I need. A guitar has a bandwidth of about 80Hz to about 5 to 6 KHz. and a little bid of harmonic distortion is not a terrible thing.
I was looking at the Hammond iron because it's cheap.
http://www.hammondmfg.com/124.htm
What do you think?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.