Anyone done a PI driver with 6V6's?

[talbany]

Max

Having a closer look at the data sheets I don't think that the data sheets are wrong. I took a look at the old GE data for a 6L6GC. The characteristics and typical operation for a push pull ab1 amp says for a plate voltage of 450V, 400V screens, -37V bias, and 5.6K load resistance you will typically get 116ma of plate current for two tubes with zero signal. That's 58ma per tube and probably OK in a push pull amp where the tubs don't conduct all of the time (like I mention in the above thread).In fact, under the same conditions screen current would be 5.6ma so if you measured cathode current you would see 63.6ma at idle.


However, I wouldn't draw any conclusions about that data unless I had the exact operating conditions e.g. 5.6k effective load resistance and peak audio grid to grid voltage of 70v.( Quite low for guitar amplifier)


Just for grins I measured an amp on the bench - with the volume on 2, I measured 200v peak - 400v peak to peak. So, you would need to limit output of the PI to ensure you don't exceed design maximum ratings of 210ma plate current (105 per tube) and 22ma screen current (11 per tube).

As you know biasing output tubes is subjective. The less than 70% method is just a rule of thumb. I don't have time to run the load lines right now. That only works as a average anyway. This type of discussion is never ending. As most of us go with what we were taught in school or a trusted mentor.
As for me, I'll stick with the rule of thumb. I don't like buying output tubes that much.

Hope this Helps

Tony

[Max]

Having a closer look at the data sheets I don't think that the data sheets are wrong.

Tony, so what do you think now based on the "CHARACTERISTICS AND TYPICAL OPERATION" specs given on page 3 of the GE6550A data sheet for the operartion of this tube in a "PUSH-PULL CLASS AB1 AMPLIFIER, PENTODE CONNECTION" concerning the idle current of 125mA specified by Alexander Dumble for two of the GE6550A tubes of ODS#0121?

Do you still think that Alexander Dumble made a fault specifying an idle current of 125mA for ODS #0121? Or is an idle current of 125 mA for two tubes @500V plate voltage in accord with the GE6550A data sheet and what you know about #0121?

Thank you for your explanations and all the best,

Max

[talbany]

Having a closer look at the data sheets I don't think that the data sheets are wrong.

Tony, so what do you think now based on the "CHARACTERISTICS AND TYPICAL OPERATION" specs given on page 3 of the GE6550A data sheet for the operartion of this tube in a "PUSH-PULL CLASS AB1 AMPLIFIER, PENTODE CONNECTION" concerning the idle current of 125mA specified by Alexander Dumble for two of the GE6550A tubes of ODS#0121?

Do you still think that Alexander Dumble made a fault specifying an idle current of 125mA for ODS #0121? Or is an idle current of 125 mA for two tubes @500V plate voltage in accord with the GE6550A data sheet and what you know about #0121?

Thank you for your explanations and all the best,

Max

Tony, so what do you think now based on the "CHARACTERISTICS AND TYPICAL OPERATION" specs given on page 3 of the GE6550A data sheet for the operartion of this tube in a "PUSH-PULL CLASS AB1 AMPLIFIER, PENTODE CONNECTION" concerning the idle current of 125mA specified by Alexander Dumble for two of the GE6550A tubes of ODS#0121?

I still think 80% (max plate) in our particular application is rather high and still run the risk of decreased tube life..I also think perhaps Alexander choose this bias setting for Tone and not so much to preserve tube life!!

Do you still think that Alexander Dumble made a fault specifying an idle current of 125mA for ODS #0121? Or is an idle current of 125 mA for two tubes @500V plate voltage in accord with the GE6550A data sheet

It is still possible to run certain tubes outside of the spec sheets recommended parameters..Just be aware this might shorten the manufactures suggested hours of operation..

However, I wouldn't draw any conclusions about that data sheets unless I had the exact operating conditions

I personally don't believe Alexander is running 0121 with the same operating parameters as what is on the data sheets (guidelines)..So it's difficult to draw any real conclusions from them..Those look to me more like Hi FI or Stereo Audio parameters where the tubes grids would not be driven as hard..Especially if 0121 is set up with the typical long tail pair driving a CF Driver section..Perhaps someone here who has run those load lines can help!

PENTODE CONNECTION"

GE 6550a Cathode and beam plates are tied internally so we run them normally as a Tetrode like a 6L6.. (unless you know something about 0121I do not)

and what you know about #0121

?
I know it's a wonderful sounding amp!!..

I hope this answers some of your questions

Have a good one!!

Tony

[Max]

I hope this answers some of your questions

Tony, yes, thanks for explaining!

The main advantage to the cathode follower directly into power tube grids is losing the effects of grid-blocking from the coupling caps recharging and causing momentary distortion when power tube grid current occurs. The amp is overall tighter (electrically) without the time constant of the coupling caps.

There also seems to be an added transparency, clarity and overall sheen to the tone with that style of driver for sure. Not sure it's for everyone, but I like it. The amp just sounds bigger and has more authority. Since the driver tube no longer has to drive a bias network or the miller effect of the multiple power tubes, I think overall bandwidth is improved a little bit as well.

Andy, thanks for explaining your opinion concerning this.

Did you - or someone else here - ever A/B the sound of a Dumble power amp with an inverter > driver > driver anode > power tube grids configuration - like the one shown in this schematic of the Odyssey power amp I posted a link to - with the sound of a Dumble power amp with an inverter > driver > driver cathode > power tube grids configuration - like in this schematic of SSS #002 I posted a link to?

What I'm talking about is some comparison like this e. g.:

SSS #001 or #002 preamp > SSS #001 or #002 power amp

vs.

SSS #001 or #002 preamp > Odyssey Concert power amp

Cheers,

Max

[FUCHSAUDIO]

I have not, but at some point would like to, when time allows.

[talbany]

Does someone here know if the loop gain and other parameters of the overall performance of the power amp might be affected to some considerable degree by the fact if - the inverter is a 12AU7 and the driver a 12BH7 and the anode of this 12BH7 driver is connected to the grids of the power tubes with a capacitor in this connection in an inverter / driver configuration like the one shown in this schematic of some Odyssey power amp: https://tubeamparchive.com/viewtopic.ph ... 153#219153

or

– the inverter is a 7025 and the driver is a 7025 and the cathode of this 7025 driver is connected to the grids of the power tubes without a capacitor in this connection in an inverter / driver configuration like the one shown in this schematic of SSS #002: https://tubeamparchive.com/viewtopic.ph ... 738#204738 ?

The block diagram shows 50uf on plates and screens..This would suggest that that amp would be rather loose than stiff..This is all of coarse the opinion of the one playing the amp and his/her frame of reference..Do you know for sure if the diagram was taken from 0121.. or another similar amp Alexander made..Perhaps the amp's filtering was changed or upgraded at one time..

Tony, what I’m sure about is this: The block diagram I posted is a transcription posted on the schematic heaven website of the block diagram you find on page 730 of the 4th edition of "The Tube Amp Book". And AFAIK this block diagram has been handwritten by Alexander Dumble. It's labelled "OD-150W, s/n 012" and signed and dated "11-11-87, HD".
Cheers, Max

FWIW: It seems they all have their own voice. The gain of the 12AU7 and BH7 as anode followers (combined) might be in the same ballpark as the single 12AX7 driver, but with a lower output drive impedance, although I have not measured that circuit. Marshall actually had this of a few of their rack mount power amps, and Mesa has it in some bass amps I think.

The main advantage to the cathode follower directly into power tube grids is losing the effects of grid-blocking from the coupling caps recharging and causing momentary distortion when power tube grid current occurs. The amp is overall tighter (electrically) without the time constant of the coupling caps.

There also seems to be an added transparency, clarity and overall sheen to the tone with that style of driver for sure. Not sure it's for everyone, but I like it. The amp just sounds bigger and has more authority. Since the driver tube no longer has to drive a bias network or the miller effect of the multiple power tubes, I think overall bandwidth is improved a little bit as well. I've used a small cap on the feedback resistor to compensate a little for that when needed. It can give a tighter more controlled and extended bass too. I've used it on 6L6's EL-34's and 6550's not on 6V6's or lower powered stuff.

Andy
Thanks for the run down..

Do you believe that the lower impedance of the CF driver (in conjunction with the Miller effect) is the main reason why that driver sounds

There also seems to be an added transparency, clarity and overall sheen

Do you also find that the CF (when driven hard) generates a large amount of 2nd order (if at all)..If so at what volume level does this seem to appear.. Rather high (gig volumes) or extremely loud (room shaking) volumes..
I assume the 12ax would be the first to go 2nd order..Correct?

Thanks a Bunch!!

Tony

[FUCHSAUDIO]

Did not map it out. We're using it in the Clean Machine, 12AX7 input and a 12AT7 or 6FQ7 as second CF tube. I think part of the clarity is the added bandwidth and speed, no capacitor being present (and the sound any cap might bring). When you run regulated lower voltage screens the grid swing and bias voltage is somewhat lower, so I think the power tubes run out of power well before the driver starting doing wacky things. Just speculation, have not measured in any great detail.

[talbany]

Did not map it out. We're using it in the Clean Machine, 12AX7 input and a 12AT7 or 6FQ7 as second CF tube. I think part of the clarity is the added bandwidth and speed, no capacitor being present (and the sound any cap might bring). When you run regulated lower voltage screens the grid swing and bias voltage is somewhat lower, so I think the power tubes run out of power well before the driver starting doing wacky things. Just speculation, have not measured in any great detail.

Thanks!!

T

[vibratoking]

Andy - totally of topic. I have seen your patent reference in your signature for a long time. I always thought it was related to amp design, but it is for security or fire detection? Just interested.

[FUCHSAUDIO]

Yes, it's for a retrofit base for smoke detectors. To replace an old system with a new one, requires new detectors and bases (especially if the manufacturers are not the same). It's a UL thing, as systems are life safety systems and UL listed as systems not devices. The retrofit base allowed a new detector, the retrofit base (which mated with the old base and the new detector), and it saved time. Instead of working with your hands above your head on a ladder to replace a base (two wires in and two wires out, plus all the shit that falls out of ceilings into your eyes...). Amazingly, it's been a rough road selling or licensing it out.

[Max]

Does someone here know if the loop gain and other parameters of the overall performance of the power amp might be affected to some considerable degree by the fact if

- the inverter is a 12AU7 and the driver a 12BH7 and the anode of this 12BH7 driver is connected to the grids of the power tubes with a capacitor in this connection in an inverter / driver configuration like the one shown in this schematic of some Odyssey power amp: https://tubeamparchive.com/viewtopic.ph ... 153#219153

or

– the inverter is a 7025 and the driver is a 7025 and the cathode of this 7025 driver is connected to the grids of the power tubes without a capacitor in this connection in an inverter / driver configuration like the one shown in this schematic of SSS #002: https://tubeamparchive.com/viewtopic.ph ... 738#204738 ?

I have not, but at some point would like to, when time allows.

Andy, I see. Brandon obviously had the opportunity to compare two SSS 150W amps concerning this question. Here’s what he reported:

Both amps had the FET volume on the front, and in one it was foot switchable(built in 82 ish originally, so the footswitchable FET is not a "New innovation" as some would like us to believe )

One of the amps (newer) had what looked to be mercury Dynaco Iron at the OT, as well as the 12AU7/12BH7 Driver setup, it also had a lower gain type preamp. It was a 150W amp with 4 6550's and definitely said "Steel String Singer" on it.

https://tubeamparchive.com/viewtopic.php?p=66244#66244

Obviously the one with the PAB engaged and the FET turned up would get a moderate amount of gain, but not even as much gain as a moderateley set tube screamer. What I loved about them is that the cleans would sustain endlessly, "Sing" if you will, and I didn't miss overdrive at all.

The one that had the Dumbleland setup with the 12au7/12bh7 driver setup wouldn't break up, however it did rattle all the walls in my house!

https://tubeamparchive.com/viewtopic.php?p=66426#66426

Cheers,

Max

[tictac]

Is there a verified schematic of the Dumbleland with the 12AU7, 12BH7 driver?

I tried that circuit but got some bad oscillation as I brought the voltage up with a variac...

The McIntosh amp that uses that PI/driver setup has the plates of the 12BH7 tied to the screen supply which is tied to the plate supply of the opposing push-pull pair....

Weird I know, just wondering if anyone has tried this circuit and have had simmilar problems...

TT

[talbany]

Does someone here know if the loop gain and other parameters of the overall performance of the power amp might be affected to some considerable degree by the fact if

- the inverter is a 12AU7 and the driver a 12BH7 and the anode of this 12BH7 driver is connected to the grids of the power tubes with a capacitor in this connection in an inverter / driver configuration like the one shown in this schematic of some Odyssey power amp: https://tubeamparchive.com/viewtopic.ph ... 153#219153

or

– the inverter is a 7025 and the driver is a 7025 and the cathode of this 7025 driver is connected to the grids of the power tubes without a capacitor in this connection in an inverter / driver configuration like the one shown in this schematic of SSS #002: https://tubeamparchive.com/viewtopic.ph ... 738#204738 ?

I have not, but at some point would like to, when time allows.

Andy, I see. Brandon obviously had the opportunity to compare two SSS 150W amps concerning this question. Here’s what he reported:

Both amps had the FET volume on the front, and in one it was foot switchable(built in 82 ish originally, so the footswitchable FET is not a "New innovation" as some would like us to believe )

One of the amps (newer) had what looked to be mercury Dynaco Iron at the OT, as well as the 12AU7/12BH7 Driver setup, it also had a lower gain type preamp. It was a 150W amp with 4 6550's and definitely said "Steel String Singer" on it.

https://tubeamparchive.com/viewtopic.php?p=66244#66244

Obviously the one with the PAB engaged and the FET turned up would get a moderate amount of gain, but not even as much gain as a moderateley set tube screamer. What I loved about them is that the cleans would sustain endlessly, "Sing" if you will, and I didn't miss overdrive at all.

The one that had the Dumbleland setup with the 12au7/12bh7 driver setup wouldn't break up, however it did rattle all the walls in my house!

https://tubeamparchive.com/viewtopic.php?p=66426#66426

Cheers,

Max

There is no compressor these amps have an unexplainable phenomenon! there is nothing strange in them at all. I think it's Dr D's mastery of lead dress and component selection, otherwise it's all regular old circuits.

Max
Not to change the topic..But reading one of Brandon's later posts..I whole heatedly agree with this statement.. I believe this too is the main reason why the Feedback City amp responds the way it does..Not to say that I am on Par with Alexander however this is still my general conclusion with respect to my version of the 102 Feedback City..
Just thought I would add!!..

https://tubeamparchive.com/viewtopic.ph ... 38&start=0

Tony

[Max]

The one ... [SSS150W - Max] ... that had the Dumbleland setup with the 12au7/12bh7 driver setup wouldn't break up, however it did rattle all the walls in my house!

https://tubeamparchive.com/viewtopic.php?p=66426#66426

To perhaps provide some perceptible impression concerning the breakup characteristics of a Dumbleland Special 150W here https://tubeamparchive.com/viewtopic.ph ... 484#130484 you'll find a clip of a Dumbleland Special 150W demonstrating the amount of breakup a Dumbleland Special 150W is able to deliver - if someone might wish to provoke breakup - at a medium rehearsal room volume together with a '62 Stratocaster and without a pedal between guitar and amp.

Cheers,

Max

[wjdunham]

I hope this answers some of your questions

Tony, yes, thanks for explaining!

The main advantage to the cathode follower directly into power tube grids is losing the effects of grid-blocking from the coupling caps recharging and causing momentary distortion when power tube grid current occurs. The amp is overall tighter (electrically) without the time constant of the coupling caps.

There also seems to be an added transparency, clarity and overall sheen to the tone with that style of driver for sure. Not sure it's for everyone, but I like it. The amp just sounds bigger and has more authority. Since the driver tube no longer has to drive a bias network or the miller effect of the multiple power tubes, I think overall bandwidth is improved a little bit as well.

Andy, thanks for explaining your opinion concerning this.

Did you - or someone else here - ever A/B the sound of a Dumble power amp with an inverter > driver > driver anode > power tube grids configuration - like the one shown in this schematic of the Odyssey power amp I posted a link to - with the sound of a Dumble power amp with an inverter > driver > driver cathode > power tube grids configuration - like in this schematic of SSS #002 I posted a link to?

What I'm talking about is some comparison like this e. g.:

SSS #001 or #002 preamp > SSS #001 or #002 power amp

vs.

SSS #001 or #002 preamp > Odyssey Concert power amp

Cheers,

Max

FWIW, I did do quite a bit of experimenting with the clean preamp of and ODS (Sorry Andy ) into a 100W ODS power amp and a 100W SSS #002 power amp using both a 12AT7 and a 12BH7 as the driver tube. I would concur with Andy that the SSS with CF drive has a bit more of a sheen on the tone. You can hear the difference in "speed" of the response when you snap the low strings. The SSS just barks at you, it does feel a bit like plugging your guitar into a bass amp. The ODS power amp without the CF seemed a bit flat by comparison, just didn't jump out. Quite subjective testing but I felt there was a subtle but noticeable difference in the response. I did not hear much difference between the 12AT7 and the 12BH7 as the driver.

The main goal of my testing was to determine whether the extra complexity of the CF driver was really worth the effort and how much a part of the overall tone and feel of the amp was dependent on it. My conclusion was that is worth keeping. At some point I plan to do an SSS #004 build and a side by side comparison of the two. They are two very different circuits and it's going to be interesting to see how different the end result is. My guess is that the feel of amp is where the biggest difference will be, something a recording most likely wouldn't expose.
Bill