EL 34 tubes in Mark 3 amp

by jrethorst Sun Jul 10, 2022 5:48 pm

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Post n°1

EL 34 tubes in Mark 3 amp

by jrethorst Sun Jul 10, 2022 5:48 pm

I would like to try EL 34 tubes in my mk 3's. Given the cautions in other posts that they won't last as long as KT 88's and the possible if partial solution of setting the bias lower, I'd still like to try. What wiring changes do I need to make? (I searched here for this info and did not find it; apologies if I missed it.)

Thank you,
John

by peterh Mon Jul 11, 2022 1:46 am

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Post n°2

Re: EL 34 tubes in Mark 3 amp

by peterh Mon Jul 11, 2022 1:46 am

jrethorst wrote:I would like to try EL 34 tubes in my mk 3's. Given the cautions in other posts that they won't last as long as KT 88's and the possible if partial solution of setting the bias lower, I'd still like to try. What wiring changes do I need to make? (I searched here for this info and did not find it; apologies if I missed it.)

Thank you,
John

pin 1 should be connected to 8 in the socket as the EL34 has no internal connection
between cathode and g3. This does not affect the use of 6550/KT88 as pin 1 is not connected
on these or in some cases connected to the metal base. This is of no concern as only low cathose
voltages are expected in this amp.

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by jwb474 Mon Jul 11, 2022 9:05 pm

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Post n°3

Re: EL 34 tubes in Mark 3 amp

by jwb474 Mon Jul 11, 2022 9:05 pm

What I would do is to get the MK2 to MK3 conversion memo that is available here and reverse the process and convert your MK3 to a MK2 and set up the bias calibration per the MK2 manual.
Unless you are really pushing the power limits of a MK3 you probably wont notice going from 60 to 50 watts.

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by **Bob Latino** Tue Jul 12, 2022 8:31 am

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Post n°4

Re: EL 34 tubes in Mark 3 amp

by Bob Latino Tue Jul 12, 2022 8:31 am

jwb474 wrote:What I would do is to get the MK2 to MK3 conversion memo that is available here and reverse the process and convert your MK3 to a MK2 and set up the bias calibration per the MK2 manual.
Unless you are really pushing the power limits of a MK3 you probably wont notice going from 60 to 50 watts.

That 2019 forum post on how to convert your Dynaco Mark II to a Dynaco Mark III mentioned by jwb474 is at the link below ...

How to convert your original Dynaco Mark II to a Dynaco Mark III

Bob

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by WLT Tue Jul 12, 2022 10:52 am

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Post n°5

MK III to MK II

by WLT Tue Jul 12, 2022 10:52 am

Completely reversing the MK III to a MK II is probable not needed. Removing the choke and installing the resistor will work but does not help with what you are trying to do. Look carefully at the revisions related to the bias circuit and maybe only do those. Or....

The adjustment pot on most Dynaco MK IIIs will have enough "reach" to actually make it so you can just put in the EL34 and adjust. Before doing the conversion (with the amp turned off) rotate the pot to max negative bias. Put in the EL34 and turn it on. Adjust the bias up so you obtain about 40mA of output for each EL34. You just have to use the 11.2 ohm value in the calcs. If you do not know how to do this we can help. If the bias comes on to high at start up then this is not working and you have to change components per the conversion document.

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by jrethorst Sun Jul 17, 2022 9:59 pm

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Post n°6

Re: EL 34 tubes in Mark 3 amp

by jrethorst Sun Jul 17, 2022 9:59 pm

I'm sorry but since I'm new at this, I don't understand some of what you've said:

> pin 1 should be connected to 8 in the socket as the EL34 has no internal connection
between cathode and g3. This does not affect the use of 6550/KT88 as pin 1 is not connected
on these or in some cases connected to the metal base.

Do you mean that, having connected pin 1 to pin 8, as the EL34 requires, that KT88 tubes could be used again without taking out the connection?

> This is of no concern as only low cathose voltages are expected in this amp.

Do you mean that the connection between pins 1 and 8 is not important, or something else?

Thank you,
John

by peterh Mon Jul 18, 2022 1:56 am

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Post n°7

Re: EL 34 tubes in Mark 3 amp

by peterh Mon Jul 18, 2022 1:56 am

jrethorst wrote:I'm sorry but since I'm new at this, I don't understand some of what you've said:

> pin 1 should be connected to 8 in the socket as the EL34 has no internal connection
between cathode and g3. This does not affect the use of 6550/KT88 as pin 1 is not connected
on these or in some cases connected to the metal base.

Do you mean that, having connected pin 1 to pin 8, as the EL34 requires, that KT88 tubes could be used again without taking out the connection?

> This is of no concern as only low cathose voltages are expected in this amp.

Do you mean that the connection between pins 1 and 8 is not important, or something else?

Thank you,
John

Pin 1 is not connected ( or connected to the metal base) in KT88. That the metal base is
connected to the cathode is no hazard as low voltages are expected.
On EL34 pin 1 is connected to g3 which must be connected to the cathode for
good function of the tube.

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by WLT Mon Jul 18, 2022 9:04 am

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Post n°8

Re: EL 34 tubes in Mark 3 amp

by WLT Mon Jul 18, 2022 9:04 am

I agree with PeterH. Look at the published schematics for the MK II (EL34 ) vs the MK III (KT88). You will see pin 1 connected to ground in the MK II but not in the MK III. Add the pin 1 to ground connections and then use the 6CA7/EL34 tubes.

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by peterh Mon Jul 18, 2022 12:02 pm

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Post n°9

Re: EL 34 tubes in Mark 3 amp

by peterh Mon Jul 18, 2022 12:02 pm

WLT wrote:I agree with PeterH. Look at the published schematics for the MK II (EL34 ) vs the MK III (KT88). You will see pin 1 connected to ground in the MK II but not in the MK III. Add the pin 1 to ground connections and then use the 6CA7/EL34 tubes.

Pin1 is to be connected to pin 8 ( cathode).

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by WLT Mon Jul 18, 2022 12:49 pm

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Post n°10

Re: EL 34 tubes in Mark 3 amp

by WLT Mon Jul 18, 2022 12:49 pm

Agreed. Pin 1 to pin 8 and then thru the small value resister to ground. Many EL34 amps do not have that resister to test across. The Dynaco and VTA amps do.

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by jrethorst Thu Jul 21, 2022 1:11 am

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Post n°11

Re: EL 34 tubes in Mark 3 amp

by jrethorst Thu Jul 21, 2022 1:11 am

> Pin1 is to be connected to pin 8 ( cathode)

Once that's done, to run EL34's, does it need to be removed to go back to KT88's? Or can it be left there?

by peterh Thu Jul 21, 2022 1:43 am

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Post n°12

Re: EL 34 tubes in Mark 3 amp

by peterh Thu Jul 21, 2022 1:43 am

jrethorst wrote:> Pin1 is to be connected to pin 8 ( cathode)

Once that's done, to run EL34's, does it need to be removed to go back to KT88's? Or can it be left there?

See post #7 above

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by WLT Thu Jul 21, 2022 9:17 am

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Post n°13

Re: EL 34 tubes in Mark 3 amp

by WLT Thu Jul 21, 2022 9:17 am

PeterH – a question for you on using the EL34 in a MK III. Assume G3 was to be connected directly to ground on a Dynaco (or VTA) amp. A small (less than 2 VDC) difference would exist between pin 8 and pin 1 on an EL34. The testing resistor is very small and I do not see how it would effect the tubes performance. Do you know what issues could or would develop? Could it be related to a slight feedback concern?

by peterh Thu Jul 21, 2022 9:51 am

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Post n°14

Re: EL 34 tubes in Mark 3 amp

by peterh Thu Jul 21, 2022 9:51 am

WLT wrote:PeterH – a question for you on using the EL34 in a MK III. Assume G3 was to be connected directly to ground on a Dynaco (or VTA) amp. A small (less than 2 VDC) difference would exist between pin 8 and pin 1 on an EL34. The testing resistor is very small and I do not see how it would effect the tubes performance. Do you know what issues could or would develop? Could it be related to a slight feedback concern?

I don't know the answer to this. I do know what philips datasheet says, connect g3 to
cathode.

I'll turn the question around : why connect g3 to ground ?

by WLT Thu Jul 21, 2022 3:10 pm

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Post n°15

Re: EL 34 tubes in Mark 3 amp

by WLT Thu Jul 21, 2022 3:10 pm

I am just inquisitive. I do not understand how something works (or may not) so I look around for answers. Without finding one I ask if I think someone may know the answer. Just me being me.

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EL 34 tubes in Mark 3 amp