5F6A/JTM45

[Geezer]

JTM
Junior tube assistant
Posts: 87
(2/9/04 7:18 pm)
 5F6A/JTM45
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I was told a 5F6A is just like a JTM45.

I've ordered the 022798 Power transformer and the 018343 Output transformer from the 5F6A parts list.

If I follow Doug's 5F6A layout and parts list do I have to change anything on the board to use KT66 tubes instead of 6L6?

Can anyone recommend a good brand and place to buy reasonably priced reliable KT66 tubes? I've noticed Doug doesn't sell KT66 tubes.

One more thing.
Should I use the 5U4G or 5AR4 rectifier tube?Is there a difference?
The 5U4G has that cool coke bottle shape.

Thanks for any help .
Tim
 
Rotting27
Junior tube assistant
Posts: 109
(2/9/04 9:37 pm)
 Re: 5F6A/JTM45
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 I've had luck with Sovtek KT66s, and they're not too expensive, as far as KT** tubes go. Should run you around 20 a pop.

I'd just pop in a 5AR4. It'll give you a touch higher B+, but really just use what you can get cheap. Or experiment with both.
 
Andy Poupart
Senior tube assistant
Posts: 114
(2/9/04 11:47 pm)
 Re: 5F6A/JTM45
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 The board layout for a 5F6A and a JTM45 are about identical. If you want to run KT66s, you'll want to beef up the screen resistors (to, say, an 8K2 shared screen resistor and individual 470 ohm screen resistors). The grid stoppers might vary, too. I don't recall off the top of my head. You might have to change the range of the bias circuit, depending on the tubes you use. You really should use a GZ34/5AR4 rectifier. It's indirectly heated, for one thing, and has a much lower internal resistance than directly heated tubes, such as a 5U4G. Those are great for low power Twins, but not 5F6As.

Note that just putting KT66s in a 5F6A design won't turn it into a JTM45. The tran

[Geezer]

Note that just putting KT66s in a 5F6A design won't turn it into a JTM45. The transformers, especially the output transformer, are absolutely key.

I've found that Valve Art KT66s are pretty reasonable. They're available all over the net.
 
JTM
Junior tube assistant
Posts: 88
(2/10/04 6:04 am)
 Re: 5F6A/JTM45
 Thanks for the all the help.


 
JTM
Posts: 91
(2/22/04 9:20 pm)
 Re: 5F6A/JTM45
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 Does anyone know if the Sovteck KT66 will fit in a weber chassis?
I've decided to try a set if they will fit the chassis after reading this:

"Sovtek KT66 - Everything you read in the review of the Sovtek 6L6WXT+ holds true for this tube. A larger than average curved glass envelope and metal base ring make this a different package than the familiar 6L6. Biasing and current draw are not issues, just the same as any 6L6GC. The sound of the Sovtek KT66 is great and usable for any application."

I don't need to change anything with the 5F6A layout to use these tubes. Correct?


www.geocities.com/retsej26/ampstuff.html






Tim

 
JTM
Junior tube assistant
Posts: 92
(3/6/04 6:58 pm)
 Re: 5F6A/JTM45
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 Ok, it's me again...
I went with the Sovtek Kt66 power tubes.
The amp is very loud, surprisingly loud. It rattles the windows with the volume set at 5 hooked to my 4x12.
It's also very clean/bright, maybe too clean for what I want.
I removed the bright cap from the volume pot to see what happened. It helped a little bit but it's still too bright for me.
I have to use my tube screamer to dirty it up to get the sound I like.
Do you think a different output transformer will help?
I've been thinking about trying one from marstran.
I've heard a lot of good things about them.

I would like to thank everyone for their input.
This has been a great learning experience for me.


Tim