Anyone done a PI driver with 6V6's?

[Bear]

I keep mulling a low powered 6V6 amp mining the Dumbleland and SSS designs to give a cleanish pedal platform for mostly home use. A lot of the discussion of the high power circuits is about the benefits of the various post-PI driver configurations. I wonder if anyone has messed with drivers in a lower powered amp.

(Probably gonna go with mosfet source followers if I build it.)

[topbrent]

I am curious about this also.

Would the follower type drivers help to drive the 6V6's in a way that would delay the onset of clipping?

SRV and/or Mayer type tones, but not at such punishing volume levels?

[Max]

SRV and/or Mayer type tones, but not at such punishing volume levels?

AFAIU you seem to be interested using a Dumbleland or SSS kind of amplifier at low volumes and without power amp distortion. I never did have any problem at all to achieve this with one of the original Dumbleland or SSS amps.

I always had lots of fun when using one of these amps even at very low volume levels when standing or sitting close enough to the speaker cabinet to get some interaction between speaker and guitar going and when tweaking the controls (tone, filters, volume, reverb, presence) in a way suited for my personal taste at such a low volume level (as you know our subjectiv perception of sound is rather different at very low voume levels and the sound of a guitar speaker, too).

And AFAIK even with a 15W amp the maximum undistorted output volume will only be around 1/2 in comparison with the undistorted output volume of a 150W amp anyway. So you might have to use the volume controls of a 6V6 amp, too, when you want to play at a low volume level. So why not just adjusting the volume controls of a 150W or even 300W amp for the same low output volume, as - if I understand this correct - power amp clipping isn't what you want to achieve anyway?

And - as you might know from looking at some of the pictures of the back of ODS #0121 posted in the internet - Alexander Dumble there specified the idle current of one pair of the four GE6550A to be 125 mA. And AFAIK a GE6550A adjusted for an idle current of 62.5 mA might operate with a very high fidelity at low volume levels in such a Dumbleland or SSS kind of power amp. And so you might be provided with a very precise amplification of whatever sound you have adjusted your Dumbleland or SSS kind of preamp for - even at very low volume levels.

Cheers and a great weekend to all here!

Max

[talbany]

SRV and/or Mayer type tones, but not at such punishing volume levels?

AFAIU you seem to be interested using a Dumbleland or SSS kind of amplifier at low volumes and without power amp distortion. I never did have any problem at all to achieve this with one of the original Dumbleland or SSS amps.

I always had lots of fun when using one of these amps even at very low volume levels when standing or sitting close enough to the speaker cabinet to get some interaction between speaker and guitar going and when tweaking the controls (tone, filters, volume, reverb, presence) in a way suited for my personal taste at such a low volume level (as you know our subjectiv perception of sound is rather different at very low voume levels and the sound of a guitar speaker, too).

And AFAIK even with a 15W amp the maximum undistorted output volume will only be around 1/2 in comparison with the undistorted output volume of a 150W amp anyway. So you might have to use the volume controls of a 6V6 amp, too, when you want to play at a low volume level. So why not just adjusting the volume controls of a 150W or even 300W amp for the same low output volume, as - if I understand this correct - power amp clipping isn't what you want to achieve anyway?

And - as you might know from looking at some of the pictures of the back of ODS #0121 posted in the internet - Alexander Dumble there specified the idle current of one pair of the four GE6550A to be 125 mA. And AFAIK a GE6550A adjusted for an idle current of 62.5 mA might operate with a very high fidelity at low volume levels in such a Dumbleland or SSS kind of power amp. And so you might be provided with a very precise amplification of whatever sound you have adjusted your Dumbleland or SSS kind of preamp for - even at very low volume levels.

Cheers and a great weekend to all here!

Max

Max
Wow!..62 ma at 450 plate voltage would be about 80% max plate dissipation at idle..IMO a bit on the high side..
At 400V max plate would be about right @ 61.5 ma..
You wouldn't by chance happen to know the plate voltage of 0121 would you?.. I heard that amp was rather stiff (from someones POV)
If the amp was indeed 400v on the plates the amp IMO should not be real stiff (as I have heard) I am also unaware of how that amp is /was filtered
FWIW..I personally like the sound feel of 6550's/KT88's at low volumes too (especially clean channel) of an ODS (Classic).. The low end feels like floating on a foodball field filled with Marshmallows..Something like that..

Does that change the tonal quality?

Yes. The low end is absolutely luscious. You feel like you're floating on a football field filled with marshmallows. And it gives a singe to the midrange that puts solos right out there. It works great for chords and solos, but especially well for slide. It's the kind of enclosure that Lindley and Lowell George used.

http://personalpages.manchester.ac.uk/s ... /Articles/

All The Best..

Tony

[Bear]

Hmm. The expense of a quartet of output tubes for power I didn't need was a factor in my thought. But that description of the 6550 outputs sounds very, very interesting. Maybe a pair of 6550's would simplify things while keeping some of that big amp character.

[Max]

You wouldn't by chance happen to know the plate voltage of 0121 would you?..

Tony, Brandon once posted here that he often found a plate voltage around 500V in the 150W SSSs he knows: https://tubeamparchive.com/viewtopic.php?p=58034#58034 And AFAIK the plate voltage of ODS150W #0121 is more or less in about the same range. If I should find out something more specific concerning this, I'll report.

AFAIK ODS #0121 technically operates perfectly and without any problems at an idle current setting of 125 mA for a pair of GE6550A as specified by Alexander Dumble on the back of #0121 on the right beside the two bias adjustment pots (see the attached picture). And AFAIK many of those who had the opportunity to use it, including myself, liked the feel and timbre of this amp a lot with this idle current setting. Todd Sharp - the original owner of ODS150W #0120 and #0121 - in "the ToneQuest Report" magazine: "The truth be known, this was a great, great amp, and I gladly return the insurance money if I could have my stolen amp back..."(#0120 was stolen after he sold #0121 - Max)"...best tremolo, reverb & Overdrive ever,..."

picture source: http://personalpages.manchester.ac.uk/s ... 20%230121/

A schematic of the power supply of an ODS 150W handwritten by Alexander Dumble you'll find on page 730 of "The Tube Amp Book" 4th Edition by Aspen Pittman. A transcription of this handwritten schematic of the power supply of an ODS 150W you'll find on the "Schematic Heaven" site (see attached document). Source: http://www.webphix.com/schematic%20heav ... _HTML.html "90's ODS Block Diagram"

And in a data sheet of the SSS150W from around 1986 the "Power Requirement" of a SSS 150W is specified to be "540VA at full output". And this (540VA) is exactly the same "Power Requiremet" as specified on the back of Dumbleland Special #009: http://www.maverick-music.com/vintage-a ... pecial-009 . And on the pictures of ODS #0121 IMO the PT and OT look very similar to the PT and OT of Dumbleland #009.

All the best,

Max

[talbany]

And in a data sheet of the SSS150W from around 1986 the "Power Requirement" of a SSS 150W is specified to be "540VA at full output". And this (540VA) is exactly the same "Power Requirement" as specified on the back of Dumbleland Special #009: http://www.maverick-music.com/vintage-a ... pecial-009 . And on the pictures of ODS #0121 IMO the PT and OT look very similar to the PT and OT of Dumbleland #009.

All the best,

Max

Max..Thanks for posting that info!!
Biasing a pair of 6550's at 125 ma with a plate voltage of 540 V (is not a good idea)would put the max plate dissipation (@ idle) around 90%..Running them this hot would seriously shorten the tube life if not red plate them immediately..So unless the laws of physics cease to exist on Alexanders bench, let's say for now, I have my doubts and recommend that no one here bias a pair of 6550's that hot

I know that amp had a rather different RP/RK (V1V2) setup than his usual non HRM amps!!


I have no doubts that amp sounds WONDERFUL!!

Have a good one!!

Tony

[Max]

Biasing a pair of 6550's at 63 ma with a plate voltage of 540 V (is not a good idea)would put the max plate dissipation (@ idle) around 93%..Running them this hot would seriously shorten the tube life if not red plate them immediately..So unless the laws of physics cease to exist on Alexanders bench, let's say for now, I have my doubts and recommend that no one here bias a pair of 6550's that hot..I don't doubt that Alexander ran the plates of his 6550's around 500/540v as this (for me) is about the sweet spot for these tubes..IMO

Tony, AFAIU the attached data sheet of the GE 6550A - the power tube ODS #0121 AFAIK was designed for - on page 1 a plate dissipation value of 42 W is specified for this tube. And with a plate voltage of even 540V and an idle current of 62.5 mA plate dissipation would be 33.75 W and only 80.36% of the maximum plate dissipation and not 93% as you write. Or is this calculation wrong?

And on page 3 an idle current of 150mA is specified as being a typical value for a pair of these tubes at a plate voltage of 450V for AB1 operation, fixed bias and pentode connection. And AFAIK this means a plate dissipation of 67.5 W for a pair of 6550A when idling.

So if ODS 150W #0121 should have the same 500V plate voltage as the one Brandon measured in some specimens of the SSS 150W and the idle current is adjusted to be 125 mA for a pair of its 6550A tubes - as specified by Alexander Dumble on the back of ODS #0121 - then AFAIK this would mean a plate dissipation when idling of 62.5 W for a pair what would be a value below the plate dissipation of 67.5 W specified in this data sheet for a plate voltage of 450V when idling as a "typical operation" value for a pair of GE6550A. Or do I misunderstand this data sheet?

However: Those more familiar with tube amp technology here than I am will be able to interpret the attached data sheet correctly and so will be able to decide by themselves if Alexander Dumble is crazy or not when specifying 125 mA as being the recommendable idle current for a pair of the GE6550A power tubes of ODS #0121 on the back of this amp.

All the best,

Max

[FUCHSAUDIO]

I am curious about this also.

Would the follower type drivers help to drive the 6V6's in a way that would delay the onset of clipping?

SRV and/or Mayer type tones, but not at such punishing volume levels?

I'm guessing that due to the lower drive requirement of both the 6V6 tube and the drive needed to achieve 20 watts or so, the CF driver would not have any real advantage. Ironically, the Ceritone SSS does not seem to have a CF driver ?!?! really ?

[ampdork]

The drive is not the only quality the post driver imparts. It also changes the phase and relationship of applied NFB.

All things being equal the post PI driver alters tone significantly regardless of the output tube used. There is more going on here then just pushing the grids...

[Max]

The drive is not the only quality the post driver imparts. It also changes the phase and relationship of applied NFB.

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".

And as the last digit of the serial number is missing, I assume, that this is meant to be the block diagram of both amps, ODS #0120 and #0121. And AFAIU what Todd Sharp told in "the ToneQuest Report", at least one of these two amps has already been built and delivered in 11/87. But of course it's possible that Alexander Dumble updated or changed the power supply of ODS #0120 and/or #0121 after "11-11-87".
BTW: I don't know of a "stiff" comment of someone who played #0120 and/or #0121.

Cheers,

Max

[talbany]

Max
Perhaps The primary impedance of the output transformer is higher than we think.. This could account for the higher bias setting..

A couple things on the block diagram of interest..

Plates are filtered after the choke (1 big ass choke)
Dropper to PI is 6k which would put the plates at around 500v

Tony

[Max]

Perhaps The primary impedance of the output transformer is higher than we think.. This could account for the higher bias setting..

Tony, perhaps Alexander Dumble for whatever reasons simply liked designing around the specs given in the data sheets of the tube manufacturers and didn't care too much about this "70% maximum plate dissipation @idle" habit of other amp manufacureres you're reporting?

AFAIU the attached data sheets and my calculations are correct, e.g. GE, Sylvania and GEC specify "Typical Operation" idle currents resulting in higher percentages of maximum plate dissipation @idle than 70%.

GE 6L6GC as an example:

Maximum plate dissipation for pentode connection (one tube): 30W

Typical operation values @idle for push-pull, AB1, fixed bias (values for two tubes):
plate voltage: 450V
zero-signal plate current (two tubes): 116mA

= 58mA for one tube x 450V = 26.1W zero-signal plate dissipation for one tube = 87% of the maximum plate dissipation.

Or do I misinterpret the attached data sheets and/or are my calculations wrong?

Here are some more data sheets for those here who might be interested checking if this "70% maximum plate dissipation @idle" habit is based on the specs given in the data sheets of the tube manufacturers: http://www.drtube.com/tubedata.htm

All the best!

Max

[talbany]

Max
I would say you misinterpret the spec sheets.. Here is why..
AFAIK..Spec sheets will often provide and not provide Plate loads- idle dissipation, instantaneous dissipation with signal applied, and average dissipation with signal applied. Idle dissipation is simply the product of the plate voltage and plate current at idle, i.e., when no signal is applied. This is easily measured and calculated. Instantaneous plate dissipation is a bit trickier, because it changes throughout the cycle of the input waveform. It is again the product of the plate voltage and plate current at any given point in time, hence the name "instantaneous" plate dissipation. Average dissipation is what really counts in most tube amp designs. It is the time-average of the instantaneous dissipations over the input cycle. Since the plate element has a certain amount of thermal mass, average dissipation is what counts. A tube's maximum plate dissipation can be exceeded at some point during the cycle, provided that it is far enough below the maximum dissipation during other parts of the cycle for the average dissipation to not exceed the tube's max plate dissipation rating (In the case of GE 6550a 42 watts) and why we use this # in our calculations....There are three factors to take into account in determining a maximum "safe" operating area for a tube: plate voltage, idle current, and the load impedance the tube is working into. The first two determine the static, or idle dissipation, and all three determine the average active dissipation under signal. As long as the maximum plate dissipation for a tube (42w) is not exceeded at idle or under all signal levels from zero signal to maximum clip, it is operating in the "safe operating area" for that tube... The maximum allowable idle current varies with plate voltage and plate load impedance (and by virtue of these two parameters, class of operation). If the plate load impedance and plate voltage are known, a maximum idle current can be determined to insure the average dissipation never exceeds the maximum specification of the tube over all output voltage swings. pec sheets are primarly used as guide lines and at times might not apply to our particular application and how we operate them to achieve our sonic goals..


Tony

[FUCHSAUDIO]

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.