6550/kt88

[aro]

They'll both run into the same load and they both have similar plate characteristic curves so the bias supply should not need to be altered. The main thing to watch for is that the screen voltage under load does not exceed 500V. If it does, it would not be a good idea to install 6550s in that amp unless you drop the power supply voltage.

Thank for the info.

The voltage was about 480, so I put in the 6550s, biased, and it sounded great. This is a 2204 clone I bought and it came with KT88s, but I want to modify it for EL34. I already have a 2204 clone with 6550s. I was afraid it was built with a transformer for KT88s, but it doesn't seem to be the case...

From what I read for EL34s it should be: 220k bias, 220k supply, 56k bias pot. Is that correct?

It's a little funny how this amp is set up. The resistor by the bias pot is 68k, the bias resistors are 100k and the bias supply is 220k. It looks like it's a little bit of everything in there...

 

[Jonathan Wilder]

Thank for the info.

The voltage was about 480, so I put in the 6550s, biased, and it sounded great. This is a 2204 clone I bought and it came with KT88s, but I want to modify it for EL34. I already have a 2204 clone with 6550s. I was afraid it was built with a transformer for KT88s, but it doesn't seem to be the case...

From what I read for EL34s it should be: 220k bias, 220k supply, 56k bias pot. Is that correct?

It's a little funny how this amp is set up. The resistor by the bias pot is 68k, the bias resistors are 100k and the bias supply is 220k. It looks like it's a little bit of everything in there...

To set up the bias supply for EL34s, the CORRECT value for the resistors on the DC side of the bias supply are 47K and 15K. The one on the AC side of the bias rectifier is 220K.

The grid leak resistors are also to be changed to 220K. You CAN run the stock 150K grid leak resistors if you want and chances are you won't hear a difference.

 

[aro]

To set up the bias supply for EL34s, the CORRECT value for the resistors on the DC side of the bias supply are 47K and 15K. The one on the AC side of the bias rectifier is 220K.

The grid leak resistors are also to be changed to 220K. You CAN run the stock 150K grid leak resistors if you want and chances are you won't hear a difference.

Thanks a lot

 

[aro]

BTW, this is the PT http://www.classictone.net/40-18023.pdf

I don't know yet how to "read" PTs... Is it a good PT for EL34s?

 

[cagamp1]

May I be a bigger asshole?

I guess you guys don't read what I read, but...

The older version of the KT88 had a max Va of 600 @ 35W.
The newer versions of the KT88 have a max Va of 800 @ 42W.

The RCA 6550 type had a Va of 600 @ 35W.
The Tung-Sol 6550 type had a Va of 660 @ 42W.
And everything I see indicates primary use as a AF power amplifier tube.

Well, not that it all matters that much anyway.

 

[Jonathan Wilder]

BTW, this is the PT http://www.classictone.net/40-18023.pdf

I don't know yet how to "read" PTs... Is it a good PT for EL34s?

Transformers aren't "designated" for a specific valve type. There just simply needs to be enough current available on the heater winding to run all of the heaters in your valve compliment.

A 50 watt Marshall requires at the very minimum 3.9 amps of heater current -

EL34 - 1.5A each
12AX7 - 300mA each

2 x 1.5A = 3 Amps
3 x 12AX7 = 900mA

This one had 5 amps available at the heater winding so it will be more than enough to run all of the valve heaters.

 

[aro]

Transformers aren't "designated" for a specific valve type. There just simply needs to be enough current available on the heater winding to run all of the heaters in your valve compliment.

I started learning about amps beginning with the input and preamp. I wanted to gather a good knowledge of those before moving over to the power amp section. I'm by no means a preamp expert, but now I understand what happens there. I just got Merlin's new book on power amps and I'm ready to move on.

Back to the issue... I was thinking (most likely wrongly) that some PTs might deliver too much voltage for EL34s. I have the feeling I'm off the mark

 

[Jonathan Wilder]

I started learning about amps beginning with the input and preamp. I wanted to gather a good knowledge of those before moving over to the power amp section. I'm by no means a preamp expert, but now I understand what happens there. I just got Merlin's new book on power amps and I'm ready to move on.

Back to the issue... I was thinking (most likely wrongly) that some PTs might deliver too much voltage for EL34s. I have the feeling I'm off the mark

It all depends on the supply design itself.

The most important part of any amplifier is the power supply. After all, the output power has to come from somewhere right?

Now...EL34s can run with up to an 800V supply. However, the screens themselves cannot take this. They're rated at about 450V max. If you plan to run the B+/plate voltage above this, a 2nd supply is needed to reference the screens to. This is what is known as a "dual rail" design. When ran in this fashion, the screen supply is typically rated to put out 1/2 the main B+ rail voltage.

On a single rail design like all Marshalls are, the screens are referenced to the same supply as the rest of the valve. In this supply design, you can have a transformer that puts out too much voltage simply because the screen grids cannot handle near what the plate can.

So it all depends on the design of the supply itself.

 

[aro]

Now...EL34s can run with up to an 800V supply. However, the screens themselves cannot take this. They're rated at about 450V max. If you plan to run the B+/plate voltage above this, a 2nd supply is needed to reference the screens to.

Oops, does it mean that since the screen voltage on this amp is 480, I can't just put in EL34s with only a few resistor changes?

 

[aro]

Actually, the plate voltage was 480, and that was measured using a Weber Bias Rite.

 

[aro]

In fact, the 220k bias resistors instead of 100k should lower the voltage, right?

 

[Jonathan Wilder]

In fact, the 220k bias resistors instead of 100k should lower the voltage, right?

The 220K grid leak resistors don't drop hardly any voltage at all. There's no control grid current flowing through them, therefore they cannot drop voltage (well...no valve is perfect so before anybody says anything, there is a VERY TINY amount of control grid current there but not enough for the leak resistor to drop any appreciable voltage).

Now...the 1K screen resistors will drop the screen voltage under max signal by a factor of about 35 volts. This will put the screens right at 455V at maximum swing. =C= EL34 valves have a 500V screen voltage rating.

Yes...guitar amps definitely push the limit when it comes to screen voltage. This is a big reason why valves get worked so hard and don't seem to last as long in guitar amps.

 

[aro]

The 220K grid leak resistors don't drop hardly any voltage at all.

Now...the 1K screen resistors will drop the screen voltage under max signal by a factor of about 35 volts.

Thanks. I definitely should start reading/studying the power amp books that I have.

Bottom line, and last question (unless you say something interesting again and I can't stop myself), would EL34s work in an amp with a plate voltage of 480?

 

[MajorNut1967]

Basically In the 50's and 60's the US and Europe where constantly one-upping each other in tube designs. So when the Europeans developed the KT-88, the US answered with the 6550.

I have to disagree with you! The 6550 has Military tube designation and developed as a Servo Driver tube for the B-52 bomb sight. It was not a commercial response to the KT88, there was no competition.
LOL Did I say that already?

 

[MajorNut1967]

The older version of the KT88 had a max Va of 600 @ 35W.
The newer versions of the KT88 have a max Va of 800 @ 42W.

So do you believe the newer versions are a more robust tube (KT88)? Then can you explain to me why they last about a week in my Major's and my 600 volt old GEC last for years?

 

[cagamp1]

So do you believe the newer versions are a more robust tube (KT88)? Then can you explain to me why they last about a week in my Major's and my 600 volt old GEC last for years?

When I say "newer", I'm still talking about the Genalex tubes from 30+ years ago. The earlier versions I spoke of were circa late 50's.

I would hope that all vintage GEC/MOV/Genalex KT88's are road worthy.
They better be for the prices asked.

As for current production KT88's ... I can only go by what their data sheets indicate...but probably would not trust them.

 

[supershifter2]

well at least i got my question answered. so for the HiWatt(HI ! HI !) DR201 that came with 6550 I should be able to run KT-88 without a problem cept for rebias. But a HiWatt DR201 that came with 6 EL34 that might be a problem. there are 2 versions of that amp.

 

[mickeydg5]

Yes. The 6550 and KT88 are interchangeable. Always check and rebias when changing power tubes.

Using KT88 in an EL34 type design may require modification.