• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Brainstorming a push-pull 6L6GC stereo amp

thorpej

thorpej

AK Subscriber
Subscriber
I don't really need another project in the queue, but I have a bunch of nifty parts in the garage and can't help myself...

I have a few nice, beefy MusicMan SS guitar amp PTs that will work well with a 4x voltage multiplier to get 395Vdc or so, with plenty of current capacity. I have a bunch of NOS Russian 6П3С-Еs (including a matched quad). I've been wanting to scratch-design a P-P amp as a thought exercise. It seems like the stars are aligned.

I'm trying to decide which direction to go. Part of me wants to do an ultra-linear output stage, but I don't really know anything about how the design process changes for UL. Another idea is to do a vanilla pentode topology, but with a regulated screen supply.

I intend to use traditional fixed bias in order to maximize my plate and screen voltages (and, really, because it's the right thing to do :-) ), but this means pretty low value grid circuit resistors (100K is the max for fixed bias according to the 6L6GC data sheet, although it seems I regularly see this exceeded -- 220K, especially in guitar amps). So I guess that means I need to be especially conscious of the amount of grid driving current available.

I'm leaning towards using a cathodyne phase inverter, simply because it's very easy to get balanced output out of them, and they're easy to DC-couple to the voltage amp stage.

The 6L6GC will have reasonably low distortion on its own in P-P, but I'd like THD to be < 1%, so I'll probably employ some NFB; that is, unless UL gets me there by way of the local feedback in the output stage. However, I'm not sure how to change the output stage voltage gain calculation for AB1 P-P (A1 SE is pretty easy).

Haven't decided which OTs to use yet, but I'm leaning towards Edcor just because the price is right and they have always served me well in the past.

I'd like to get at least 25W/ch; I don't think I have enough voltage to get much more than that.

So, I'm soliciting ideas...
 
Register to hide this ad
I vote for regulated screen supply and local feedback a la Schade or E-Linear if you already have an UL transformer. And if you are adventurous, you can even get more output with class AB2, but it requires a beefy CF/SF driver. Anyway, whatever we can think of, it's all been done already, do a search and you'll get literally thousands of proven schematics. It's just a matter of deciding what you want to do...
 
Having recently built an amp with an 807 push pull UL connected output stage, it definitely can deliver the sonic goods. Output transformers make all the difference in the world for high frequency ease and extension. The frontend on mine is a common cathode voltage amp direct coupled to a differential pair inverter/driver (aka Mullard 5-20 style). Mine puts 400V to the center taps, uses 6.6K primaries with 43% screen taps, and puts out a clean 20 watts/channel. I added 10.5 dB global feedback. The bass isn't quite as tight as I would have liked, but in return the midrange is really quite glorious.
 
For the application I'm looking at, Transcendars are in the same price ballpark as the Edcors, it's just a matter of having the primaries I'm after... haven't drawn up my load lines yet, but I'm estimating I'll have around 375V for the plates once resistive losses in the filtering are accounted for.

The topology I'm leaning towards at the moment is common cathode voltage amp DC-coupled to a cathodyne inverter, with the inverter outputs RC-coupled to low-mu driver stages, a'la Williamson, with UL connection.

@kward, which transformers did you use?
 
Hammond 1650H. They are reasonably good transformers--not the best, but also reasonably priced. I think the 1650F and 1650H are the sweet spot in the Hammond lineup. I have not tried Transcendar. I hear their SE stuff gets excellent reviews.
 
There is a long thread on the same type of amp over at diyaudio that has been going on for about 5 years. They settled on LTP with CCS and George is coming out with a universal PCB soon.
 
Last edited:
jazbo8 said:
That's just blasphemous.:D
Click to expand...

Haha, fair enough. There's a schematic for the particular one I'm thinking about in an old Acrosound transformer catalog. Anyway, thanks for the link back to the old thread... the Eico HF87 topology looks like it fits the bill, too... but I'm trying to wrap my head around the phase inverter... is it being cathode-driven by the other half of the 6SN7 (the direct-coupled AF amp stage)? And a little puzzled by the unequal plate resistors on the 6SN7...

http://vintagevacuumaudio.com/schematics-manuals/eico/eico-hf-87-schematics.pdf
 
If you can live with only 25 watts/channel, I would try to snag some old stock Heathkit AA-100, HK A50K, or HK TA-5000X output transformers, or anything in that era that was used with a 6L6/7591/7868/7355 type tube with about 6K to 6.6K primaries. That would make one sweet little 6L6 amp.
 
thorpej said:
And a little puzzled by the unequal plate resistors on the 6SN7...
Click to expand...
That is what is commonly referred to as the Mullard phase inverter, which is a variant of the long-tailed pair that you may be more familiar with. The different plate resistors are used to balance the two outputs.
 
The second triode in the differential pair (or bottom triode as drawn on the Eico schematic) is driven from it's cathode, which isn't quite as efficient as driving it from its grid like the first triode, hence the need to balance the outputs with a slightly larger plate resistor. In this style, the second plate resistor is usually made to be about 20% larger than the plate resistor of the first triode. Some people will make that second plate resistor varaible in some fashion so as to be able to dial in more exact AC balance. Others use a CCS in the cathode and make the plate resistors equal (higher tail resistance forces more equal balance). Still others do both. I've tried both approaches, and ultimately I prefer the variable resistance in the plate load of the second triode--I find it a little easier, if not as elegant as a CCS.
 
Ah, yes, I know why the plate resistors are often unequal in a LTP, I just didn't recognize it as such the way that it was drawn (too used to Fender schematics, I guess). I like the adjustable load idea, @kward... in this case, a garden variety 1/2W 5K pot would probably do the trick.

As far as transformers, I actually already have a pair of Edcor 6.6K 25W OTs... when I bought the transformers for 8802 clone I built for my daughter, I picked up a second set to eliminate the Edcor lead time when I get around to building one for my son... though, I learned quite a bit while building that and would definitely entertain making changes to that circuit, which would free up those transformers. The danger, of course, is them each being mad at me if both amps aren't EXACTLY THE SAME :)

I think this weekend I'll do some load line analysis of the HF87 circuit .. I'm wondering why they run the ECC83 at that operating point and then bypass the cathode... my perhaps naive initial impression is that they could run it at a higher voltage operating point to achieve similar gain without bypassing the cathode, which would tend to reduce distortion in that stage, right?
 
Last edited:
Oh, never mind.... I see why they run the ECC83 at that voltage... it's all about being able to set the operating point of the direct-coupled LTP correctly.
 
For a good tutorial on how to properly design a modern push-pull amplifier, you can check out Miles Prower's write-up here. He also manges to dissect and destroy a few light-weight, dilettante designs along the way.
 
jazbo8 said:
For a good tutorial on how to properly design a modern push-pull amplifier, you can check out Miles Prower's write-up here. He also manges to dissect and destroy a few light-weight, dilettante designs along the way.
Click to expand...

In fact I asked Miles for a copy of his "Vixen" 807 push pull amp schematic before I got heavy into the design of my own 807 type amp. I can't say mine is based too much on his work, but it was interesting to see how he came about solving the issues of blocking distortion, getting enough drive, etc. His design reminded me a bit of Morgan Jones' Crystal Palace.
 
@babblefish - I am sorry, but that's probably one of the worst modern designs in recent years.:no::thumbsdown:

@kward - yes, both the CP and Miles' designs share the same design philosophy.
 
Last edited:
@babblefish - given all the design choices available, it seems that Mr. Heran somehow managed to make some rather odd ones, does more "wrongs" make it "right"? May be, but why do things that way? The issues with the SRPP stage and the CCS cathode bias have already been covered by Broskie, so I won't rehash them here. That leave us the self-split output stage, which as we all know, was nothing but a cost-saving measure reserved for the low-end consoles/radios, it's a "simple design" alright, but good? Not really...
 
You must log in or register to reply here.
Back
Top Bottom