Marshall 1959SLP KT-88 OT and PT upgrade?

CEG85

CEG85

Active member
Hello, Would appreciate some advice on this topic.
Want to replace my Mercury Magnetic O100JM OT to a Mercury Magnetic MMO-100M OT to accept KT88/6550’s.
Currently just have the MM OT with stock PT.
Question is do I need to upgrade my stock PT as well?
I’m open to other brands, doesn’t necessarily have to be MM.
Thanks
 
The main issue with the PT supporting an output tube change is the heater current draw of the new tube type.
EL34's draw 1.5amps per tube.
KT88 / 6550 draw 1.6amps per tube. So an increase of 400ma across a quad. About the same as adding another 12ax7 preamp tube to the amp, which guys have been doing for years without a problem.
What you NEVER want to do is try to run EL34 / 6550 / KT88 in an amp designed for 6L6's.
6L6 heater current is .9amps per tube, so going from a quad of 6l6 to a quad of EL34 increases heater current draw by approx 2.4amps, which WILL smoke your PT or burn out your heater traces on the pcb.
This is actually a common occurrence with guys trying to run EL34s in the OG evh 5153 100watt head.
 
As everyone else has already stated if the power transformer is designed to run EL34 tubes then you can run KT88/6550 tubes no problem but usually it's the impedance of the output transformer that Marshall had to consider when they began shipping their amps with 6550's instead of EL34's in the 70's. You do have to adjust the bias circuit to provide about 10 more volts of negative bias voltage to run the KT88/6550's in their operating range, there is a tone of info out there on how to adjust the bias circuit from EL34's to KT88/6550's.

That's why Marshall decided to switch to them for the Marshall Major due to the amp 200 watt rating and the 6550's for American market when the EL34's were getting damaged in shipping from the UK to the US as they would work with the impedance range of the output transformers already spec'd out for EL34's. FWIW the KT88 in some opinions sounds and responds kind of like a larger EL34 still with accentuated mids but with more lows and detailed highs.

I'm running a stock Dagnall PT pulled out of a Marshall SLP plexi in my 79 Marshall 2203 and I have been running 6550 tube's for years now, the bias circuit has been adjusted to provide the negative bias range for 6550'/KT88's. I am also running a MM Axiom MM0-100M output transformer in the same amp and I believe this is MM standard Marshall plexi /superlead output transformer which I guess is optimized to run 6l6/6550/KT88 according to MM. But I do believe the stock Marshall plexi output transformer can run KT88/6550's no problem even though the impedance is not optimal for the KT88/6550. I don't think you absolutely have to change out the output transfomer if you don't want to....
Marshall didn't and the KT-88/6550's run just fine in old Marshall's and they should with the O100JM. MM omitted the primary impedance of the MM0-100M on their site for some reason but I did some digging and it appears as though the primary impedance of the MM0-100M and 0100JM both are 1.7K ohms so I don't think you really need to change it out unless you really want to. Maybe someone else can do some sleuthing on the primary impedance of the MM0-100M to see if I am correct or not.

Spec Sheet

MMO-100M​


Marshall | 100 Watt | Output Transformer
Optimized for 6L6 / KT88 / 6550 tubes — 1.5″ stack. Uses Grain-Oriented Iron

Marshall​

https://www.google.com/url?sa=t&rct...usg=AOvVaw09spJ3fGaGLM_UGxyoq4i4&opi=89978449
Mercury Magnetics
https://www.mercurymagnetics.com › make › marshall





MMO-100M. Marshall | 100 Watt | Output Transformer. Optimized for 6L6 / KT88 ... **Standard 1.7 kOhm primary impedance version of the TXOP-00001. Bolt hole ...
30-day returns



The MM O100JM appears to be a dagnal C1998 clone of some sort which should still be able to run KT88's IMHO.

Spec Sheet

O100JM​


Marshall | 100 Watt | Output Transformer
Plexi — 1.5″ stack — DE #C 1998 — 1.7 kOhm primary impedance — 3″ x 2 1/2″ bolt hole spacing
 
Last edited:
You're supposed to use 100k bias feed/grid leak resistors with KT-88. You probably have 220k like most el34 amps.

Also keep in mind if you change those resistors you'll cut some saturation, and low end. You can add the low end back with larger coupling caps.
 
harddriver said:
As everyone else has already stated if the power transformer is designed to run EL34 tubes then you can run KT88/6550 tubes no problem but usually it's the impedance of the output transformer that Marshall had to consider when they began shipping their amps with 6550's instead of EL34's in the 70's. You do have to adjust the bias circuit to provide about 10 more volts of negative bias voltage to run the KT88/6550's in their operating range, there is a tone of info out there on how to adjust the bias circuit from EL34's to KT88/6550's.

That's why Marshall decided to switch to them for the Marshall Major due to the amp 200 watt rating and the 6550's for American market when the EL34's were getting damaged in shipping from the UK to the US as they would work with the impedance range of the output transformers already spec'd out for EL34's. FWIW the KT88 in some opinions sounds and responds kind of like a larger EL34 still with accentuated mids but with more lows and detailed highs.

I'm running a stock Dagnall PT pulled out of a Marshall SLP plexi in my 79 Marshall 2203 and I have been running 6550 tube's for years now, the bias circuit has been adjusted to provide the negative bias range for 6550'/KT88's. I am also running a MM Axiom MM0-100M output transformer in the same amp and I believe this is MM standard Marshall plexi /superlead output transformer which I guess is optimized to run 6l6/6550/KT88 according to MM. But I do believe the stock Marshall plexi output transformer can run KT88/6550's no problem even though the impedance is not optimal for the KT88/6550. I don't think you absolutely have to change out the output transfomer if you don't want to....
Marshall didn't and the KT-88/6550's run just fine in old Marshall's and they should with the O100JM. MM omitted the primary impedance of the MM0-100M on their site for some reason but I did some digging and it appears as though the primary impedance of the MM0-100M and 0100JM both are 1.7K ohms so I don't think you really need to change it out unless you really want to. Maybe someone else can do some sleuthing on the primary impedance of the MM0-100M to see if I am correct or not.

Spec Sheet

MMO-100M​


Marshall | 100 Watt | Output Transformer
Optimized for 6L6 / KT88 / 6550 tubes — 1.5″ stack. Uses Grain-Oriented Iron

Marshall​

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwi95cepnuWCAxWQg2oFHSKxAFsQFnoECBAQAQ&url=https://www.mercurymagnetics.com/make/marshall/&usg=AOvVaw09spJ3fGaGLM_UGxyoq4i4&opi=89978449
Mercury Magnetics
https://www.mercurymagnetics.com › make › marshall





MMO-100M. Marshall | 100 Watt | Output Transformer. Optimized for 6L6 / KT88 ... **Standard 1.7 kOhm primary impedance version of the TXOP-00001. Bolt hole ...
30-day returns



The MM O100JM appears to be a dagnal C1998 clone of some sort which should still be able to run KT88's IMHO.

Spec Sheet

O100JM​


Marshall | 100 Watt | Output Transformer
Plexi — 1.5″ stack — DE #C 1998 — 1.7 kOhm primary impedance — 3″ x 2 1/2″ bolt hole spacing
Click to expand...
Really appreciate the detailed response.
Thanks alot! :cheers:
 
CrazyNutz said:
You're supposed to use 100k bias feed/grid leak resistors with KT-88. You probably have 220k like most el34 amps.

Also keep in mind if you change those resistors you'll cut some saturation, and low end. You can add the low end back with larger coupling caps.
Click to expand...
Thanks for bringing that up,:2thumbsup: I forgot to mention that. You can go as low as 82K with .1uf coupling caps will get your low end back (this corrects for the R/C constant change) and be the most stable configuration for KT88's/65550's instead of the 220K/.022uf coupling caps used for El34's.

I used 82K/.1uf coupling caps in my 6550 amp build I mentioned and then I swapped out the .1uf caps for .047uf cap to tighten the amp up some more. That amp had a ton of low end with the 82K/.1uf configuration.

Marshall split the difference and used 150K resistors and did not change the coupling caps, they were left as is at the .022uf value and they didn't have any issues. They were afraid the 6550's might go into runaway bias but that never happened and the KT88/6550 will run just fine with the 150K/.022uf cap values. Crazynutz's 100K/.1uf or .047uf might be a good compromise all around.

FWIW my Splawn modded 79 Marshall is running 220K/.022uf coupling caps there, and Scott Splawn built it that way knowing 6550's were being used. It has run fine for years since 2003 since he rebuilt it and I haven't felt the need or desire to change out the 220K's to 82-150K values. I also recommend a cool bias no more than 50% of dissipation bias with KT88's/6550's as they can get smeared sounding and lose their detail with a hot bias like in the 70% range like some people use on EL34's. Anymore these days I run all my stuff at 50-60% max. My Ed spec amp I run at 70% but it is variaced down alot and I pull two tubes as well.
 
harddriver said:
I also recommend a cool bias no more than 50% of dissipation bias with KT88's/6550's as they can get smeared sounding and lose their detail with a hot bias like in the 70% range like some people use on EL34's. Anymore these days I run all my stuff at 50-60% max. My Ed spec amp I run at 70% but it is variaced down alot and I pull two tubes as well.
Click to expand...

Honestly I could never stand hot bias, just too harsh sounding for me. I much prefer cool bias.
 
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