JTM45 Build with 6P3P output tubes and GZ34

[Cqwet Dbdfte]

More voltage in the power supply affords a higher idle voltage, with correct grid bias.
But your coupling caps may have to be upgraded, and plate resistor has to be checked for max volts, two in series may be in order.
I have a bag of 10W 100k w.w. resistors that I have used for very low nose applications , replacing metal film resistors that became noisy, in a $10,000 power amp.
Some tubes are better spec'd for higher voltage, like the 6SN7 and similar.
However, if you want max voltage swing without very high +B, I would suggest using a constant current source like a pair of DN2540's, and should be set to a linear current level on the grid/plate chart.
This will extract the highest linearity of a tube. Great for driving low mu power tubes.

 

[Spencerleehorton]

checked over my work thoroughly this time and found a few missing wires!! got the variac and current limiter on and fired it up again!!

works like a charm!!! so im using 6P3P as output transformers, the only problem so far is the bias of these, the closest i can get them is 13mA and 25mA, so not ideal, but actually sounds great.
Have gone through and checked all voltages and all are within a few volts here and there.

Just need to build a cabinet for it now!! and burn it in.

 

[Spencerleehorton]

only problems i can find so far is that the presence pot wasnt working so changed and is fine now and the amp will need a PPIMV as this amp is pretty loud!!! even on half volume, im sure it will sound so much better with the correct OT.

 

[StarTrucker]

the closest i can get them is 13mA and 25mA

You can add a second bias pot and bias them individually to match exactly

 

[Spencerleehorton]

Where would I add it?

 

[Spencerleehorton]

Do you mean to separate the two 220k resistor which attach to the the 56k,15k, 10uf and put another 56k with 50k trim to gnd on both 220k?

 

[merlin]

Yes something like that

 

[Spencerleehorton]

Ok perfect yes that’s what I was understanding it as, what mA should I aim for with the 6P3P tubes?

 

[Spencerleehorton]

https://www.tedweber.com/bias-calc/

I’ve put 6L6gc into calculator and it about 430v, will check again but bias comes out at about 48mv

 

[Spencerleehorton]

Crude drawing time!!


This look ok?

Sorry forgot to put other gnd off 50k trim and 10uf +!!

 

[Accelerator]

@Spencerleehorton​

I’ve put 6L6gc into calculator and it about 430v, will check again but bias comes out at about 48mv

Man, why are you trying to make your life so miserable cheap-y and difficultly stupidly any time?...

The 6P3P output tubes which are related to the 6L6G output tubes have Maximum Limiting Rating Value the 330 DC Volts (Shuguang) ~ 360 DC Volts (Tung-Sol), so they will have a very short (but brilliant…) carrier with a lot of “red-plated” anodes in your 430 DC Volts JTM-45 Amplifier…

If the 28 ~ 30 watts that you got by the 6P3P output tubes as pentodes or the 14 – 15 watts that you got by the 6P3P output tubes as triodes are “Too Loud” for you, then you have to consider for a matched pair of 6V6S JJ Platinum Matched output tubes (€22,61 each) connected at the 8KΩ (A-A*) tap of your Rediospades type JTM-45 Output Transformer, that will withstand the 430 DC Volts of your JTM-45 Amplifier and tame down the volume…

 

[Spencerleehorton]

Thanks for that, oops!! I have some EL34s here or like you say get some nice 6v6S.
If I use the EL34 will I have to change anything majorly?

 

[Spencerleehorton]

What would be great is a little check list so I could work out what I need for EL34, 6L6S, 5881 or KT66 tubes.
I would like to try a few different types before I settle on what I think works best.

BTW with the variac at 180v and current limiter on (light bulb 100W) the amp sounded amazing!!! With these 6P3P tubes!!!

 

[Spencerleehorton]

Btw I’m not too worried about it being too loud as I will put a PPIMV in there along with a fx loop

 

[Matador]

You can also do bias balance. Just change your 220K resistors to a fixed resistor in series with a pot. Remember that you need some resistance to ground because otherwise there is no DC current entering the grids of the tubes (no through the capacitors from the phase inverter), and adjusting the pot won't change the voltages at the grids.

 

[StarTrucker]

Just change your 220K resistors to a fixed resistor in series with a pot.

220k in your diagram becomes 100k in Matador's.


Or if it makes more sense to you to start from scratch, take the bias circuit here, duplicate the section with the 22k, 56k, and 10uf electrolytic cap and send it through a 220k resistor to the tube grid. You will have one wire lead from each 10uf cap. Connect that to a 220k resistor that is flying off your tube socket. Each tube will be biased by its own 22k pot. They're drawn as rheostats here, but a pot with pin 1 to ground and pin 2 to the 56k and pin 3 floating is fine.

I did something like this on my Black Flag JTM 45/100 for all four tubes and it was fine.

If I use the EL34 will I have to change anything majorly?

As long as the tube can handle the B+, you will need usually need to change the output transformer for different types of tubes, and probably screen resistor values. See what other 6L6 amps are using for screens. Sometimes you can use a EL34 transformer with 6L6/6V6 tubes because the reflected impedance on the primary is higher. I believe JCM800s do this. Make sure the transformers can handle the wattage you're putting through them.

Check data sheets to see what the plate impedance (RpK or Anode resistance sometimes) of your tubes are at 400v B+ in class AB (Not Ultra-linear).

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It's common to hear that for practical purposes small amps are "better" these days --> studios (guitarists are still bringing 100 Watters in for session work though), in ear monitors, better mic'ing in venues "lets you use smaller wattage amps". But remember it's YOUR amp for you to enjoy and share your music through. Go big with the El34s and high wattage if that's what you love, or 6v6s if you want to tone it down would still be cool. Even at low volumes my super lead feels bigger to play through in my hands than lower wattage fenders, even if technical voltage levels coming out of the amplifiers are the same. If you want something different, you can always build more amps.

 

[Spencerleehorton]

Think my best option is to get a pair of 6V6S, will order some.

 

[Spencerleehorton]

Looking at the spec sheet it looks like the 6V6S bias is 45ma?
I am using a 100 watt output txf at the moment so it looks like I could use some EL34s?
Will also check bias as I might have a matched pair and not need to do the separate bias.
Hopefully don’t need to change anything else for the EL34 as the 6V6S will take a couple of weeks to get here.
Maybe worth build a smaller voltage version to use these 6P3P tubes?

 

[Spencerleehorton]

I actually have another mains transformer which is 2 x 170vac and has several 6.3v windings from the line 6 spider amp so this could possibly be a consideration for another plexi amp for these 6P3P tubes?

 

[StarTrucker]

I actually have another mains transformer which is 2 x 170vac

If you mean 170-0-170 then yes with a diode bridge rectifier you should get more than enough voltage (340v) to run most power tubes. I believe JCM800s were down in this voltage range. The early JTM amps ran a high B+, the 100 Watters used a 560V B+ power transformer.

I am using a 100 watt output txf at the moment so it looks like I could use some EL34s?

Look at plate dissipation on your spec sheet, which gives you the wattage of the tube. Generally, output power is a little different than plate dissipation, but it gives you the idea. EL34s are a 25 watt tube, so two of them in your amp will give you about 50 watts, not even close to your transformer max spec. When you overdrive them they dissipate a little more as square waves make more power than sine waves. A 100watt Marshall is 4 El34s, but at full power can give you about 130 watts of output power.