Push-Pull 6V6 amp with some Changes

[andresbianco]

Hi all,

I'm starting a new project and have some doubts and maybe you could help me or give me some hints!

I want to build a tube amp and I already have some parts and "wanted" specs:


1- I have a OT push-pull 25 watts with several output possibilities (1,25 2,5 5 7,5 10 14 18 30 75 100 125 300 350 400 450 500 ohms), I tested it and measured 6,2 Kohms on the primary. (also, i can use a mis-matched output to obtain different impedances at input)

2- I have a power transformer with 6,3VAC and 230-0-230 VAC

3- I would like to use a pair of 6V6s in push-pull Class A

4- I want a single-ended capability (shut off one grid for even order increase)

Here are my questions:

1- How to bias the power tubes/valves to achieve Class A push-pull?
2- How to estimate the needed input swing to drive the output stage?
3- How to design the splitter to accomplish this?

Thank you for your help in advance!


Note:I split this post from an old topic to keep the conversation unified. Please follow the link to the old topic for info on using a push-pull OT for single-ended operation. --HBP

[shooter]

For 1 & 3 you might start with sluckeys ampeg j-12B jet schematic.  That should give you a good start on cathode bias and the PI.  For #2 use the tube data sheet as a start n tweek for the amount of drive you want.

hope that gets you started.

[PRR]

> 1,25 2,5 5 7,5 10 14 18 30 75 100 125 300 350 400 450 500 ohms

Is it a real Boat Anchor? I wish I knew you 7 years back.

> How to bias the power tubes/valves to achieve Class A push-pull?
> How to estimate the needed input swing to drive the output stage?

Seems like this has been answered before?

Seems like it would be a very popular question.

Seems like the guys who Invented the 6V6 probably had to post the answer, or lazy designers wouldn't even get started and 6V6 sales would be small.

Franks: http://www.mif.pg.gda.pl/homepages/frank/vs.html

Search for "6V6".

Frank has been adding lots of data-sheets and it has become a bit of an art to pick a good one first try. The top of the list is *usually* older, which may be more concise. The tube's original maker is authoritative; however he does not have the early Ken-Rad sheet. Tung-Sol is usually reliable if dry.

Tung-Sol 6V6 sheet 1965

First page shows "Single Tube". This might guide your half-lame mode, or maybe not.

Second page says "Push Pull Class AB1". This IS class A for any practical purpose. Hair-splitters should stay off the stage.

All necessary data is there. The more-likely condition is 8K load and 285V B+ for 14W out.

> power transformer with ...230-0-230 VAC

230V AC will peak-rectify out near 320V DC. Many people over-Volt their 6V6s this much. Conservatively you would plan a dropping resistor to get from ~~320V down to 285V with 74mA-105mA current. 35V/100mA is 330 Ohms 3.5 Watts (use 7W or higher). With practical losses you may not get 320V, or you may get more. Expect to change this to fit. (A handful of 100r 5W resistors is a good tool.)

The datasheet shows nominal fixed-bias. Lab Test conditions. In Real Life you should ALWAYS run resistor-bias Whenever Possible. We want 19V at 74mA-105mA. In fact we typically take the idle current, 75mA, and assume the amp will be OK at full roar. 19V/74mA is 250 Ohms on my abacus. (And I think I've seen it on some two-6V6 plans? Not joking... never trust a calculation that doesn't look right against known-good plans!) This resistor's power-rating should be based on the Full Roar current, at least 105mA. 250r*0.105A= 26.3V, 26.3V* 0.105A= 2.8 Watts, use at least 5W (and 10W is not wrong, or costly).

I've typed too much for this humidity. Surely you are not the first to build a two-6V6 amp? It is not a new past-time. So there must be driver plans somewhere. It is not clear if you want a guitar amp (this place's focus) or a hi-fi amp; the choice guides what plans you will seek-out and plagiarize study.

[PRR]

> a bit of an art to pick a good one first try.
> tube's original maker is authoritative; however he does not have the early Ken-Rad sheet.

Ah, found a next best thing. Radiotron 1944.
http://www.mif.pg.gda.pl/homepages/frank/sheets/084/6/6V6GT.pdf

This has a 250V *12K* condition not found on other sheets. It requires a heavy hot screen bleeder, and oddly the plate current *drops* at full power. I suspect this is a "conversion condition" to sell 6V6 into 6F6 designs who already had a stock of 10K-12K OTs.

[jazbo8]

Since you are posting here, I will post my reply to your question over at MEF, perhaps others will jump in to give you more feedback:


I did not go through your steps in detail, but I think you got the basic concept. However the chart that you used is ONLY applicable for Eg2=250V, it B+ = 325V, then all the plate curves need to be shifted up by a constant, the amount of which can be found in Radio Designer's Handbook (RDH4). Going back a bit, you said:

"Maybe I'm a little confused: even in Class A (push-pull) some even-order harmonics are cancelled out due the balanced OT primary, right? I just aim to get them back."

It is worth noting that ONLY the 2HD of the output stage itself is cancelled by the push-pull action, whatever is generated by the previous stages still appears on the output. Slightly OT, it seems that you want to get more 2HD or perhaps higher distortion in general, yet you choose Class A push-pull, which generates the lowest distortion of all the output topologies...