Separate Cathode Bypass Resistor/Cap per 6V6

[greg]

Hello all,

I have a cathode biased, dual 6V6 amp which currently has the standard shared bypass R\C.

I'm interested in giving each 6V6 its own set so I can perform the Carr trick of reducing power by 1/3 or so by bypassing the caps in the R\C setup.

I tried this in a little SE amp and it worked pretty darn well - better than I expected, anyway. It didn't change the tone too much, but made it considerably lower volume.

For Push-Pull, do I understand correctly that I need to double the value of the resistor and cut the cap value in half?

Thanks in advance,
Greg

[Alex R]

- half the R value per valve, yes, but same value cap would be fine as that's not too critical.

But can't you just bypass the single shared cap?

[greg]

OK - I cut the resistor value in half. Thanks.

As for your question:

I would have thought that you could simply bypass the shared resistor, but as I understand it from Carr (from a Tonequest article on the Vincent amp), you don't get the same gain reduction when they are shared.

Basically, he states that the gain of a preamp tube can be drastically altered by removing its cathode bypass cap and that you can do the same thing with a power tube if they are separated - it doesn't make sense to me why this would be different (I simply don't know nearly enough electronics theory).

For that matter, I don't understand why separating the two would help to bias the tubes better, but I have seen that recommended several times. I'm about to do that on a SE parallel 6V6 amp per the recommendation of David from Allen amps.

As for this Push Pull amp, I'll try it both ways and will report the results (assuming that someone doesn't first come in an either prove this or dispell this theory).

Thanks,
Greg

[MWJB]

Removing the bypass cap of a shared PP cathode resistor also drops gain significantly.

Possible advantages to separating the cathode resistors include being able to run different types of tube on the same socket, on your parallel SE 6V6 amp you could swap 2x6V6 for 1x6L6/5881 if each socket has it's own cathode resistor.

In push pull it would allow you to run just 1 power tube SE (we'll overlook the lack of an air gap for now), again subbing 2x6v6 for 1x6L6/5881 would work (OT primary current rating allowing). See the Victoria Regal. Or, it can simply allow you to eliminate a suspect tube quicker after a blown fuse - where 2 tubes share the same resistor, fitting one tube only will hugely underbias (too much current) the installed tube & probably destroy it.

I'm not familiar with the Carr amp, but perhaps removing the bypass cap at one socket only, results in a more subtle gain drop & a degree of PP mismatch that lowers headroom? I am speculating here though. Several other manufacturers do employ various methods of deliberately unbalancing the PP arrangement to reduce headroom.

[greg]

Hmm... Regarding your comment:
"Several other manufacturers do employ various methods of deliberately unbalancing the PP arrangement to reduce headroom"

That's a new one to me. Can you give me an example of amps which do this? Sounds kind of interesting. My goal with this build is to make my amp a gig amp and a practice amp (by simply lowering the overall amplification) - but this sounds kind of cool to. I suppose I'll be putting a switch for each tube if this can produce interesting sounds...

Thanks,
Greg

[ricach]

I enjoy reading threads like this because it really helps me learn more about the electronics involved. The conclusion I gathered from needing to halve the resistance value is because its the current you want to remain the same going to each tube. But why? I was under the assumption - from something I read elsewhere - cathode bias has changing current needs - and that voltage is what remains constant. Is this incorrect?

What amp currently runs SE into parallel output tubes? I'd like to compare the schematics of that to a PP schematic. Thanks.

[MWJB]

Peavey Valveking VK112 texture control, Kinder HarpKing, Kendrick Texas Crude Harp amp...an SF Fender with bias balance can be deliberately unbalanced with regard to plate current to achieve a mismatch, some harp players like a 12DW7 in a Fender style long tail PI as another route to the same end.

The Kendrick & the Kinder unbalance the PI (by different methods, Gerald Weber's harp amp tricks are well documented..google it. I'm not at liberty to discuss Kinder's methods), I'm not sure of the mechanism by which it is achieved with regard to the PV. Victoria Regal is really 2 SE outputs driving a common speaker load, but you can fit any combination of 1 or 2 octal output tubes. There are other stereo SE amps that can do the same trick, but drive 1 OT & speaker per tube.

I'm not sure that a PP mismatch will be a lot of guitarists cup of tea in a lot of cases, because an unbalanced PI after a point kills fidelity, some harp players like this, as it can appear to thicken tone & calm brittle highs. With guitar you want some of these highs for chime.

As I said an easy way to turn a 15-20W PP, cathode biased, medium gig amp into a practice amp would be to sub 2x6V6 for 1x6L6 (as long as you have individual cathode resistors per tube socket AND the OT will take the current for the 6L6) and give 5-8W?

Switching in/out cathode bypass caps will afffect gain & dynamics, not so much power...as long as you still have enough gain to overdrive the amp. I'm not familiar with the so i don't really know what the intention is, but splitting the cathodes frees up a lot of options.

[MWJB]

It's not particularly current but search Gibson GA8 schems.

Depends on bias conditions but for cathode bias it's more other way round to your assumption, the amp can't supply any more current and the voltage drops when large demands are made of the power supply.

[greg]

What amp currently runs SE into parallel output tubes? I'd like to compare the schematics of that to a PP schematic. Thanks.

Do a search for the Angela Instruments Super Single Ended amp. You'll see a schematic diagram there. I used that and incorporated it it to a 5E2 Princeton - but tweaks are in progress.

So far, I'm not sure that I'm diggin' the parallel option like I hoped. The reason being that I was told that A) I should give them their own set of cathode bypass caps & resistors (still need to order them, but I don't expect this will change much other than giving me the flexibility to use different tube types as mentioned earlier in the thread).

Second, I was told that to properly match the primary OT impedence with my 8ohm speaker when using 2 6V6s in parallel, I should switch to the 16 ohm tap on the OT. When doing this, I found that I really wasn't getting an increase in output like I was hoping (which, for this particular amp, was my goal - a dedicated home practice amp with the option for more volume for rehearsals.

I see a theme here - I'm trying to build flexibility into each simple amp I build instead of enjoying it for its intended purpose. I haven't hit tone-&-flexibility nirvana yet, but hell, it is certainly bringing me into more knowledge now that I am straying from the schematic and getting more curious.

As always, the input from you guys has been invaluable and it is appreciated.

Greg

[pdf64]

See this link for investigation into effect of cathode cap in common cathode resistor P-P output stage - Peter

http://www.aikenamps.com/cathcap.pdf

[greg]

Hello all.

I wanted to provide an update on my build and pose a few more questions (likely, the same questions, unfortunately).

I've employed the split bias R/C on my amp - 'maybe' successfully...Hopefully, I'm just not understanding how to read the measurements I'm getting.

Prior to separating the bias on the 2 PP 6V6s, I tested appx 42ma between the CT of the OT and Pin 3.

The plate is at appx 390vdc and the bias resistor is 300ohm.

My understanding is that if I want to separately bias them to the same draw, I'd need to cut the existing bias resistor value in half and just keep the same cap value.

Using this information, I employed 2) 150 ohm resistors and powered it up. My measurements now read appx 80ma from the CT of the OT to the plate of each tube. It sounded the same, so I expect that I simply need to half this reading to tell me what either tube is drawing. For grins, I doubled the original bias resistor value for each tube to get a reading that matched 42ma when measuring from the OT CT to the plate, and the amp sounded obviously wrong.

So, Question 1 is simply why does the reading differ just because each tube is biased separately?

Question 2:
On a related note from this thread, I went with separating the 6V6 bias hoping to get a fairly substantial decrease in volume when switching out the 2cathode bias capacitors. This has proven not to be the case.

In the case of the Carr amp I mentioned (the Vincent), they state that it drops it from 33 watts down to 7 watts. They also state that this is a Class A amp, so I don't know if the fact that mine is definitely not running at Class A voltages is coming into play here.

I have verified that I am in fact switching the grounded side of these caps to and from ground, but it really isn't making much difference in output level or sound (I can detect some added low end with the caps engage, but again, it is not substantial).

I'm using 1000uf caps here based on some information that I read over at the 18 watt boards which state that you can increase the cathode bias caps up to 1000uf to 2000uf safely. Apparently, it increases low end up to a point, but will then stabilize the biasing somewhat to get rid of some of the razziness of the distortion.

Can anyone comment on what I am experiencing here?

Thanks in advance,
Greg

[MWJB]

If your shared cathode resistor was 300ohms, you should give each socket 600ohms to retain the same plate current. If you only have 1x 6V6 at each end of the OT you are getting that 80mA on each tube...I'd turn the amp off.

I would be very surprised if it really sounded the same as 42mA per tube. Until you get experienced in remembering sounds (not something humans are naturally very good at) it can be very hard to hear pre/post mod differences, especially when the mod takes a significant amount of time to install.

Switching in/out the caps affects gain, headroom & tone, it won't significantly affect W (unless you can only get the power you need with everything wide open with the caps engaged.

The Carr amp is not achieving that difference in W by switching in/out cathode bypass caps. The only way you would get a difference like that is by pulling/turning off a power tube (unless you are using some kind of additional attenuation).

"I have verified that I am in fact switching the grounded side of these caps to and from ground, but it really isn't making much difference in output level or sound (I can detect some added low end with the caps engage, but again, it is not substantial)." Again, I'd be very suprised if this were the case.

Usually, people find that response is stiffer with bigger power tube cathode bypass caps, often after around 100uf there isn't a lot of point in going bigger. I'd try 22uf, 47uf, 100uf and see what you liked best.

[greg]

Hmm. Now I am truly a little confused.

Earlier in the thread, AlexR posted:
"- half the R value per valve, yes, but same value cap would be fine as that's not too critical".

But if I understand MWJB correctly, I need to double it. That's certainly no problem, but as I said, I did end up putting in 2) 750 ohm resistors (what I had lying around), which brought my current readings to around 33ma, as I recall and the amp had a seriously low output with a cold-biased crappy sound. This being the case, is the difference between 33ma and 40ma that drastic?

I'm hoping that I haven't just made a wiring mistake.

Either way, being that this hasn't provided me with the desired result (switchable wattage), I'll just go back to the shared bias arrangement and enjoy it as it is.

And, MWJB - you are of course correct in that my ears don't remember what it sounded like before I was biasing it at 80ma. Glad I haven't played for more than 2 minutes since going down this path.

Thanks for the info - this is the best sounding amp I've made yet and I'd hate to ruin it!

BTW - the amp in question is more or less a Z Ef86 Channel tied to a Top Hat Club Deluxe output section. It's my first build with 6V6s and also my first attempt at straying from a known-good schematic (well, OK - I tied together a couple of known-good schematics). It really holds together with a full, Fendery sound much better than I had hoped considering that I'm using a tiny Hammond M100 reverb amp transformer. It has surprisingly good headroom - much more than all of the EL84 amps I've built.

So thanks for all of the help everyone - I never would have gotten into this excellent mess/hobby without you.

Greg

[MWJB]

"as I recall and the amp had a seriously low output with a cold-biased crappy sound. This being the case, is the difference between 33ma and 40ma that drastic?" Look at 40-ish mA as your upper limit, within that limit bias to your taste.