Need help tweeking a schematic for 6k6 amp

[RyanThomas13]

For my next project I want to build a push pull 6k6 amp, this will get me pretty close to my target of 10 watts output. My plan is to have a plexi style preamp. I found this schematic (see attached) on Mr. Huss' page which I think could be a good starting point, but I need help converting the power supply for use with 6K6 tubes. And advice on picking transformers, for now I'd like to stick with SS rectifier.

Any help would be much appreciated.

Plexi6V6c.gif

[Phil_S]

You didn't say what you'd like to see for plate voltage on the 6K6's. Instead of the 325-0-325 PT, I might go down to 275-0-275. I didn't do any calculations, so this is a SWAG. There isn't that much difference between 6V6 and 6K6. Aside from the choice of PT, I'd probably build it as shown.

[bepone]

seeing the curves on lets say 300-320DC plate voltage.. around 12k Raa transformer

[bepone]

or take any 20W 6600 ohms transformer and connect 8ohm speaker on 4 ohm secondary, and ths will be 13.2k - ok

[cdemike]

Agree regarding PT being a prior question and that 275-0-275 makes sense if you're trying to run the 6k6 within datasheet spec. Do you have strong preferences either way in terms of low- or high-voltage sounds? If you go lower B+ (probably a good idea), I'd also suggest considering revising your B+ string to keep preamp voltages closer to what's typically seen in these amps. That said, I tend to prefer my Marshalls to run really high preamp voltages, but I know that's not everyone's thing (esp. not if you're after the EVH sound). I tend to like 10k as the first dropping resistor rather than the usual 20k, so I might try 5k, 2.2k or 1k between the screens and PI nodes.

I might also play with filtering values, based on the lower power section. Part of what makes an early Marshall what it is is power supply sag, so I'd want to experiment with 16uf or even 8uf on the screens.

Based the datasheet, 100k is as high as you'd want to go for output grid leaks, so I'd consider even going lower, depending on what your goals are in terms of headroom and responsiveness.

[RyanThomas13]

Yes, I should have clarified that I have no intention of trying to run the 6k6's on 6v6 voltages. I think if I remember right 315v is the max?

Sound wise I would prefer plexi style 50w/100w EL34, but lower volume obviously. I understand normally EL84 is a good choice when trying to downsize from EL34 but that is still more headroom than I want. Also trying to stay away from tube rectifier sag, or low uF filter sag. In general I want to keep things as "tight" as possible and I will probably play around with the NFB resistor to find that sweet spot between loose/crunch and clean/tight, I may even just install a variable NFB so I don't have to choose.

So for the PT we agree it makes sense to change from 325-0-325 to 275-0-275, still 180mA?

Please see my re-attached schematic = is this better?

Also I know the choke is probably overkill, but I think it adds something so I'm going to keep it, do I need to keep the 5k 5w resistor in parallel with it?

Plexi6k6.png

[cdemike]

Yep, 315 is the maximum rated plate voltage. Assuming you proceed as-discussed, you're still well above that limit (I estimate your B+ to be about 385V with full-wave rectification). I know people run 6V6s all the time way over the rated maximum, but I'm not that familiar with 6k6, so I'll hold off on weighing in on what impact that might have, longevity, etc. FWIW, Mark Huss' schematic clearly also runs them far above their rated maximum, and I know he is active sometimes here -- maybe he'd be able to weigh in on whether he'd do 325-0-325 again.

You'll get varying opinions on whether EL84 is a better substitute for EL34 at lower volume. I've read people saying 6V6 is actually closer to their ears, and others who say EL84 is closer. I suspect the main differences you'll encounter in trying to get tubes to sound like scaled-down EL34s have more to do with what OT you're able to run based on output tube selection. Alternately, have you considered just running EL34s at low B+? I owned a Marshall SV20, which sounded great and had lower volumes using EL34s by just running them at very low B+ values (260V is a value I see online from people who have opened theirs up). The simplest solution to getting lower-volume EL34 sounds may just be to use EL34s, in other words. Just my $0.02.

If you're trying to avoid sag, then I'd definitely suggest revising the B+ string to keep preamp voltages high. I'm not really sure what's going on with the 5k resistor in series with the choke outside of a potential effort at reducing B+. In keeping with the higher preamp B+ and headroom piece, I'd omit that. Higher preamp filtering (50uf at every node) would also make the amp tighter. It'll depend on your transformer choice, but higher filtering at the screens and for the reservoir may also be worth investigating (50uf each would also be my suggestion for a starting point). What's the rationale for lowering the output section grid stoppers?

I'm really like variable NFB setups. My last plexi build has a 250K pot wired as a variable resistor in series with a 27k NFB resistor. Generally it makes sense to lower the value of the NFB resistor when reducing the amp's output since the signal used as negative feedback becomes smaller with lower amp output. 100k thus seems large, but if the goal is a low-headroom amp, that's the right direction anyway. Depending on chassis choice, faceplate real estate, etc., you could even just have high and low value NFB resistor values switched via a push-pull pot.

[RyanThomas13]

"What's the rationale for lowering the output section grid stoppers"
I mis-read your first post, I see now you were referring to B+ string, you want instead of 22k-10k-10k something like 10k-4.7k-4.7k for dropping resistors correct?

And 275-0-275 is probably still too much, you think 250-0-250 is good, or still lower?

I did think about running low V on EL34's and I may circle back to that, but for now my curiosity about 6K6 amp is stronger.

[cdemike]

Gotcha -- yeah, I don't think there's a ton of difference between the grid stopper values listed anyway, so that makes more sense. Regarding the B+ string, yeah I think that'd be a solid starting point. You may want to tweak values once you get the amp built. You might even want to go with more radical values like 1k for some, but it'll really depend on where you land for a PT.

Speaking of PTs, I'd recommend checking this page out: https://www.hammfg.com/electronics/tran ... eferer=968

The specific circuits we're interested in are the capacitive input load full-wave bridge and full-wave rectifiers (3rd to last and 2nd to last, respectively). In any event, if you're trying to stay below the datasheet-spec maximum B+ value, you'll need 222.77V or lower if running strictly by the math, i.e., 111-0-111 or just 222.7V if you opt to use a non-center tapped PT. The actual B+ will be lower than 315V if you nailed exactly 222.7V since there are inefficiencies in the diodes, so the value I'd aim for would be in the range of 110 to 120V on each side of the CT. Assuming you bias to 70% at 315V for a full 8.5W each, your output section should draw about 38mA. That 8.5W estimate is probably too high if I'm understanding the volume goals of the project correctly, but I'd go with an over-spec transformer if you're hoping to avoid power section sag. So with two 12AX7 preamp stages and the PI, your total current draw is approximately 60mA. With a center-tapped PT and full-wave rectification, that should be your minimum. If you decide to go for a full-wave bridge rectifier, you'll need about 97mA (see the same Hammond reference). You may have difficulty finding transformers with those specifications, especially with primary taps rated that low, but they're out there. If you're open to toroidal PTs, Antek makes some that are pretty close to the B+ spec and WAY above your current requirements, so power supply sag won't be an issue if your use larger filter capacitors: https://www.antekinc.com/as-05tc250-50v ... ansformer/
https://www.antekinc.com/as-05tc200-50v ... ansformer/

Note that with those transformer, you'll need to wire the secondaries in series with the white and yellow leads twisted together and grounded to form a center tap if you want to keep the bias circuit the same (that style of bias circuit only works with a center-tapped transformer).

Edited: realized the first transformer link I shared couldn't be wired to be center-tapped.

[RyanThomas13]

Would the Hammond 263AX work?

[cdemike]

Yeah,263AX should give you a B+ of ~280V. The problem with 263AX is that it doesn't have a 6.3V tap. 269AAX also has a 200V HT secondary but has a 6.3V secondary for the heaters, so that'd be the better option. 261M6 is rated for 215V @ 269mA and has a 6.3V secondary, so it'd also be even closer to the B+ range you're looking for and definitely will have no appreciable power supply sag with strong filtering. You have a lot of options in terms of PTs, including if you're open to something like a Zener diode on the CT to drop B+.

[brewdude]

I built an amp using a pair of 6K6GT’s.

I chose the Edcor XPWR224 PT
https://edcorusa.com/products/xpwr224-5 ... 5570&_ss=c
270-0-270v @ 125mA
3.15-0-3.15v @ 3A
0-5v @ 2A

The info I’ve recorded using a 5Y3GT rectifier:

310v @ plates
285v @ screens
-24v bias @ grids
16.9mA/17.8mA
5.3W/5.5E

And, the Edcor GXPP10-12k OT
https://edcorusa.com/products/gxpp10-12 ... 1699346619
10W
12k primary
8R secondary

My build began as a 10w single channel, no tremolo, brown deluxe.

It’s more of a 10w single channel blonde baseman now… well wrt the tone stack any ways.

[RyanThomas13]

@brewdude thanks for this info, for now I'm leaning towards SS rectifier (at least until I hear what it sounds like) though I realize tube rectifier would lower my voltages and open up different doors.

@cdemike, when you mentioned 261M6 I thought why do I recognize that part#? Sure enough I went and dug thru my parts tub and I already have this PT!

So I guess that's a no brainer. I also have a Hammond 1750PA Output Transformer, I think this pair was going to be for an 18w project that I never got around to.

1750PA specs:
18W 70hz-15khz
8400ohm CT
4/8/16ohm taps

Would this OT work?

[RyanThomas13]

So assuming I can use both transformers I have on hand. I edited the schematic with all mentioned changes. Most notably B+ filter caps and dropping resistors and switching to full wave bridge rectifier.

Does everything here look ok for a starting point?

We talked about later maybe going more drastic in lowering the dropping resistors, what should be good target voltages for the preamp? Something like (moving left to right) v1 200v - v2 250v- pi 300v?

Plexi6k6.png

[cdemike]

Schematic looks good! I'd be concerned your primary impedance is too low (the datasheet echoes bepone's suggestion for 12k ohms) -- I'm not sure how much mismatch is acceptable, but a quick Reverb search shows that, for example, a Blues Jr OT can be found for about $25 before shipping. That has a primary impedance of 6.6k, so it'd need to be wired such that the speaker impedance is double the labeled value, as per bepone's second post. You may be able to find similar or better deals locally, on Ebay, etc. I'll reiterate that I'm not sure how much primary impedance mismatch is acceptable, but it seems relatively inexpensive to spec the OT closer to spec (esp. if you factor in relative costs associated with long-term tube life and that 6k6 aren't in production anymore AFAIK).

A few notes:
-I recognize it's an artistic choice and very unlikely represented in either yours or Mark Huss' actual built amps, but I feel obligated to say it's wise to have the outlet ground wired to its own dedicated chassis ground lug separate from any other ground points.
-I'd redo the standby switch so that it only handles AC, rather than the shown DC setup, in order to increase switch life by avoiding arcing inside of the switch.

The other suggestions I have are much lower in priority and more reflections of my own preferences, but you may agree with some of them:
-I prefer to have a HT fuse and a fuse on the output section cathodes. It's much cheaper and easier to replace fuses rather than amp components (esp. OTs and power tubes) if there's a power surge, a component failure, a wiring mistake, etc.
-Speaking of build economics, I think it's cheaper in the long run and makes more sense on a technical level to run two separate bias feeds for independent bias of each output tube, particularly since 6k6 aren't in production anymore, so matched sets might be harder to come by in the future.

Otherwise it looks great! Excited to see this come together.