Vintage Inspired Jtm45/100 Block-end Build

[valvestorm]

Not a full blown museum piece, but a nod toward the vintage with some modern updates... the goal being to get the tone! Thanks to Neikeel for his help getting this right!

Custom Back panel


Block-end chassis begged from another builder


Preamp section with switchable 100pf bright cap. Using some 110k plate caps to dial things in.



Re-worked the power section to avoid switch pops and to accommodate Larry Grounding.





NOS GECs

 

[Geeze]

Beauty! Love KT66's. When do we get to hear it?

Russ

 

[Codyjohns]

Does it sound as good as it looks??

 

[Ron W]

Great Scott, Robin! That looks magnificent. Clips please...

 

[neikeel]

Beautiful work, how do you find the bright cap switch works for you?
Guess I am going to have to get my JH project out of the cupboard now! Been gathering the parts for 2 years now, I was not able to prise a block end chassis from George but I the Dickinson one is old style chassis.

 

[coldengray]

Great work Robert. Looks fantastic.

 

[v846]

Beautiful build ! Love those GEC's , what are you running for ECC83's ?

 

[fusionbear]

Great Work Robert!!!

What transformer brand and voltage is your PT?

 

[valvestorm]

Beautiful work, how do you find the bright cap switch works for you?

I like the bright cap at certain points on the volume rotation, but not others... also depends what pedals are in front. Glad I made it switchable.

Beautiful build ! Love those GEC's , what are you running for ECC83's ?

They are Harma retros

Great Work Robert!!!
What transformer brand and voltage is your PT?

Both transformers are the Metro versions made by Heyboer. The PT (1204-43) is running about 495v at the plates.
https://store.metropoulos.net/collections/transformers

 

[neikeel]

Both transformers are the Metro versions made by Heyboer. The PT (1204-43) is running about 495v at the plates.
https://store.metropoulos.net/collections/transformers

That is interesting how he now specs lower voltage on his PT now. I had one of the first batch he made up (before he went into production). I am pretty sure these were 530v on the plate with the UK taps.
I would be interested to know what your PI and preamp valve plate voltages are. My 7206 replica has dual voltage options, one for the high and low voltage options, but I have been saving my last quad of NOS matched GECs for it, so I will be ok running the high voltage option if it sounds best.

 

[fusionbear]

Hello Robert, to piggyback on neikeel's comments, you may want to adjust the 82k resistor to a lower value between the choke and the PI in order to get the right voltages in the rest of the circuit. I suspect you may be as low as 178 B+ at the node that feeds V1. My build has the Metro/Heyboer PT and with 2-KT88's, I am at 510B+. I settled for 20k on mine.

 

[neikeel]

I think there is a typo above as the resistor between choke and PI is 8k2, I have seen vintage ones (well x2 PA 45/100s) with 4k7 5w resistors there, it may have been a popular mod or they came that way stock??
Raising the PI voltage to design levels will give it more punch but that might be at the expense of V1 tuning (there are 110k plates on V1 and V2).

 

[fusionbear]

I think there is a typo above as the resistor between choke and PI is 8k2, I have seen vintage ones (well x2 PA 45/100s) with 4k7 5w resistors there, it may have been a popular mod or they came that way stock??
Raising the PI voltage to design levels will give it more punch but that might be at the expense of V1 tuning (there are 110k plates on V1 and V2).

Aah, my bad, thank you for correcting me, it is a 8k2. The early schematic shows an 82k so I assumed:

http://www.drtube.com/schematics/marshall/1959t-66.gif

In that case, you may want to experiment with going up in value. I am happy with 12.9k on mine.

 

[fusionbear]

Here is a 67' PA version that I worked on with the 8k2 Iskra between the Choke and PI :

 

[coldengray]

Carl - what is the board filter cap value on that '67? A friend of mine has a 67 PA head that has a HUGE modern cap on the board, I suggested he replace it with one of the 32/32 reformed Erie caps that Valvestorm sells. But I know the value is inconsistent, sometimes 16/16 and sometimes 20/20.

 

[Big Mike]

lovin that. Beautiful work!!!

 

[coldengray]

Glad to have you here, Big Mike!!

 

[Big Mike]

Gracias amigo. Been away too long. Nice to see a dedicated 'classics' forum. FUN stuff.

 

[neikeel]

Carl - what is the board filter cap value on that '67? A friend of mine has a 67 PA head that has a HUGE modern cap on the board, I suggested he replace it with one of the 32/32 reformed Erie caps that Valvestorm sells. But I know the value is inconsistent, sometimes 16/16 and sometimes 20/20.

Not Carl here but it is a single 16uF on there, typical for the 65 and 66 JTM45/100s running KT66s. Next in the line in early were typically 32/32 with 10k dropper between, slightly browner sound. Unusual to see the single cap with that cap board.

 

[fusionbear]

Carl - what is the board filter cap value on that '67? A friend of mine has a 67 PA head that has a HUGE modern cap on the board, I suggested he replace it with one of the 32/32 reformed Erie caps that Valvestorm sells. But I know the value is inconsistent, sometimes 16/16 and sometimes 20/20.

That is a single 16uf. Standard on this circuit