Hoffman's plexi50 mod for kt88?

[AxelKay]

Hello everyone! I have a (quick) question: can I use kt88's on the plexi50 instead of the stock el34, by changing the bias tap from 220k to say 180k? Is it possible taking in account that I'll be using the corresponding plexi50 Hammond set of transformers? do I need to change anything else? or it simply won't work? thanks for your time.
Best regards! :)

[SILVERGUN]

Hello everyone! I have a (quick) question: can I use kt88's on the plexi50 instead of the stock el34, by changing the bias tap from 220k to say 180k? Is it possible taking in account that I'll be using the corresponding plexi50 Hammond set of transformers? do I need to change anything else? or it simply won't work? thanks for your time.
Best regards! :)

Give us Hammond #s
The short answer is no.
Both xfmrs will have to pass more current than the standard 50watters are designed for.

[DummyLoad]

at one time, marshalls made for the US market were equipped with 6550 - so i'd say yes, with a minor tweak of the bias supply to get the B- down to 6550/KT88 territory and swap of the grid-leak resistors. this was done during the Unicord years.

usually the 6550s were ditched in favor of the EL34 and the grid-leak resistors and the bias mod reversed.

the differences:
1) a 15K series dropping resistor in the bias supply for 6550 tubes and 27K for EL34 tubes on the 100W models with a dedicated bias supply secondary.
2) the grid-leak for EL34 was 220K and the grid-leak for 6550 was 82K for the 100W models 
3) later 50W models changed both the bias supply scheme and the grid leak resistors. EL34 models were equipped with 220K grid-leak and the USA 6550 models were equipped with 150K grid-leak. for the bias supply, the B+ tap resistor was changed to 120K with 6550 and 220K for EL34, the bias trim resistor for 6550 is 47K and 56K for EL34s respectively. 

schema's attached for comparison of the differences.

--pete

[PRR]

Putting the big tube in place of the medium-big tube should work. Check bias (similar but all tubes vary).

The Ampeg VT40 was sold both with EL34 starter tubes and 6550(short KT88) "for professional use". IMHO asking 60W of EL34 the way Ampeg ran the VT40 was asking for eventual trouble ("mine" developed a EL34 short which ate the power transformer). But hey, it out-lived its warranty. OTOH 6550 @ 60W is not abuse.

[Ritchie200]

Back in the early 70’s Marshall made the Park 75. There are no schematics that I am aware of - and I have scowered the net looking. From what I have found, it is a basic 1987 50 watt Marshall with KT88’s and a tweak to the bias.  They did have a massive lay down PT so who knows what voltages they were at. However basic 50 watt EL34 voltages for the hot JMP’s of the day would have given you 75 watts out of a pair of KT88’s. I played one with matching 4x12 back in the late 70’s at a pawn shop and it sounded wonderful. I wish I would have had money to buy it as they are rare birds worth a pretty penny today. Please post pics and sound clips of your finished project!!!


Jim


https://www.vintageguitar.com/15454/park-75/#

[Ritchie200]

Unmolested


Jim


https://www.thegearpage.net/board/index.php?threads/park-75-ltd-ready-to-be-shipped.1598739/

[AxelKay]

Give us Hammond #s
The short answer is no.
Both xfmrs will have to pass more current than the standard 50watters are designed for.

https://www.hammfg.com/electronics/transformers/guitar/jcm-800-50w
But obviously they have A LOT of options!

Edit; Fixed quote. Willabe

[AxelKay]

Back in the early 70’s Marshall made the Park 75. There are no schematics that I am aware of - and I have scowered the net looking. From what I have found, it is a basic 1987 50 watt Marshall with KT88’s and a tweak to the bias.  They did have a massive lay down PT so who knows what voltages they were at. However basic 50 watt EL34 voltages for the hot JMP’s of the day would have given you 75 watts out of a pair of KT88’s. I played one with matching 4x12 back in the late 70’s at a pawn shop and it sounded wonderful. I wish I would have had money to buy it as they are rare birds worth a pretty penny today. Please post pics and sound clips of your finished project!!!


Jim


https://www.vintageguitar.com/15454/park-75/#

I'm not necessarily after the plexi50 tone per se, I love the layout but yeah, I would not mind a hot version such as the jmp. I'm very open to suggestions, it's just the plexi is a very neat and "tidy" build if you will. Haven't paid the necessary attention to the jmp1 or anything, but a tight high gain preamp, in conjunction with the kt88 bottom and high crispiness is what I'm after. 50 60 70w...ok by me by the way!

[AxelKay]

at one time, marshalls made for the US market were equipped with 6550 - so i'd say yes, with a minor tweak of the bias supply to get the B- down to 6550/KT88 territory and swap of the grid-leak resistors. this was done during the Unicord years.

usually the 6550s were ditched in favor of the EL34 and the grid-leak resistors and the bias mod reversed.

the differences:
1) a 15K series dropping resistor in the bias supply for 6550 tubes and 27K for EL34 tubes on the 100W models with a dedicated bias supply secondary.
2) the grid-leak for EL34 was 220K and the grid-leak for 6550 was 82K for the 100W models 
3) later 50W models changed both the bias supply scheme and the grid leak resistors. EL34 models were equipped with 220K grid-leak and the USA 6550 models were equipped with 150K grid-leak. for the bias supply, the B+ tap resistor was changed to 120K with 6550 and 220K for EL34, the bias trim resistor for 6550 is 47K and 56K for EL34s respectively. 

schema's attached for comparison of the differences.

--pete

Ok so if what eurotubes claim for example, that the kt88 are a drop-in replacement to the 6550 is true, by doing that 50w build is going to be fine supposedly! Correct? Is there any layout for that? I have a major dyslexia issue and it takes me three times than average to do a layout to schematic and vice versa.

[Ritchie200]

Yes they are a drop in replacement if your bias circuit will accommodate. 

Ok, if you want to go down the rabbit hole, check this out.  Lots of possibilities and Tubenit furnished full blown schematics.  THE best KT88 amp known to man....

Jim

https://el34world.com/Forum/index.php?topic=17548.0

[Ritchie200]

BTW, the Park 75 and the Major are nowhere near the "plexi" tone.

Jim

[AxelKay]

Ok, if you want to go down the rabbit hole, check this out.  Lots of possibilities and Tubenit furnished full blown schematics.  THE best KT88 amp known to man....

Jim

Wow great read!!! I think I'll follow the major minor iii 50w schem and do the build! By the way is there any layout accompanying it? It takes me ages to go from layout to schema and vice versa 

[Ed_Chambley]

Well Jim, I have an orange park 75. It runs 6550, good el34s, 6L6gc, but it usually has winged c KT88. The amp is basically a 1987 circuit. The main difference is it has a volume knob between inputs and that huge cap on the board.


I ran some 6V6s JJ and they worked fine, but it was a test.


I prefer Mullard EL34 xf2 and did make a pre master like a 2204. It sounds just like a 1987 plexi channel jumping needed.


Voltages vary a little, power tubes around 430 to 445 vdc according to load. Never had any transformer problems.

[PRR]

> if what eurotubes claim for example, that the kt88 are a drop-in replacement to the 6550 is true

KT88 is designed to be a direct replacement for 6550 except it is an inch taller. Bias requirements are the same, within the +/-20% variability of all tubes.

At some point I though we were talking EL34. This is a lower-power tube (though ample in the 50 Watt output zone) and does have slightly different bias needs. The 6550/KT88 will be more reliable at 50W output, but the EL34 has a different distortion flavor so is often used "for tone".

[Ritchie200]

I still say a KT88 in a 6550 "setup" will be a shivering KT88 that will sound like dung unless you can swing some additional bias - which usually is not available without mods.  We also must consider the grid leak in a perfect world!  Definitely a compromise with 6550/KT88 swaps IMHO.  It completely changes the character of the Major when you run 6550's - and not in a good way.  I know the metal guys like the 6550, but I think it is the least "musical" of all tubes, again IMHO.

ED!!! I didn't know you had a Park75!!!!  If you were to draw out the schematic, you could make a fortune!  That amp has so much mystery and mojo associated with it, you could sell the copies.  Do you like it better than the Marshall 50 or 100 watters?  How does yours distort?  You added a MV mod?  How dare you!!!!

Jim

[Ed_Chambley]

I still say a KT88 in a 6550 "setup" will be a shivering KT88 that will sound like dung unless you can swing some additional bias - which usually is not available without mods.  We also must consider the grid leak in a perfect world!  Definitely a compromise with 6550/KT88 swaps IMHO.  It completely changes the character of the Major when you run 6550's - and not in a good way.  I know the metal guys like the 6550, but I think it is the least "musical" of all tubes, again IMHO.

ED!!! I didn't know you had a Park75!!!!  If you were to draw out the schematic, you could make a fortune!  That amp has so much mystery and mojo associated with it, you could sell the copies.  Do you like it better than the Marshall 50 or 100 watters?  How does yours distort?  You added a MV mod?  How dare you!!!!

Jim

Jim, it is a 1987.  The one I have is a collectable simply because a different factory transformer.  It distorts well with EL34, sustains with KT88, cleaner.A friend who passed away at 51 owned it.  His wife GAVE it to me.  He was a drummer.  Died from respiratory failure during sleep due to opiates.   Sad!

[Ritchie200]

Sad story too often repeated. I’ve lost old bandmates over the years due to “outside” influence. Always a tragic waste.


So if it is a 1987, what is that magical mongo of a cap in the middle of the board?  Also, the bias circuit HAS to be different...right?


Jim

[Ed_Chambley]

Sad story too often repeated. I’ve lost old bandmates over the years due to “outside” influence. Always a tragic waste.


So if it is a 1987, what is that magical mongo of a cap in the middle of the board?  Also, the bias circuit HAS to be different...right?


Jim

The cap is a 32uf/400 feeding the preamp tubes.  The bias circuit with 2, EL34 is exactly like my Marshal JMP50.  Sure, you have to change a resistor to run KT88, but that is about it. 

[shortfuse]

I built this amp back in 2010 and put JJ KT88's in it with no problems.  I used Dougs Marshall 50w power and OT transformers.  I just changed the bias range resistor.

[Ritchie200]

Shortfuse
Too cool! How did you like the end result? Sound clips?


Another useless factoid...  always been rumored that Page ran KT88’s in his 100watt Marshall’s. There is definitely a different, cleaner, sound coming from those super leads. When Zep did the reunion thing a few years back, there were camera shots from behind the amps. It sure looked like a row of KT88’s back there.


Jim

[shortfuse]

Well I do not have sound clips.  I have a few friends that play professionally and one of then used it while we were jamming one night and he had to have it.  So I sold it to him. 
I will say that when I was running it before the hot switch mod, it had a ton of punch and more bottom end than ever needed.  Especially when channels were jumped.  Paired with 4X12 loaded with G12H speakers the sound was awesome.  After the hot switch mod it was like a JCM800 on steroids.
I have been away from here for a few years, neck surgery and working out of town.  But was just thinking about building another and surfing the forum when I came across this thread.
Here is the link to my original build it was my 2nd build.  https://el34world.com/Forum/index.php?topic=10466.msg97676#msg97676