Amp tinkerers and people who aren't afraid of 700 volts or so in here....

Moderators: Slowy, Capt. Black

User avatar
BY jvpp
#807562
Slowy's Jansen 4/twelve special received quite a good reception at the Bollix gearfest. So much that I am tempted to build one in a similar vein. I say similar, as it is an old amp, and lacking some modern safety features and also someone has been in there before I poked around!!! A few capacitors and the odd resistors have been changed, so impossible to draw up an original circuit diagram. As I was allowed to perform non destructive investigation only, I measured the resistors in circuit. I found quite a few had drifted to much higher values, so rebuilding the amp with the values as indicated by the colour bands, may slightly change its character perhaps. More knowledgeable people will hopefully come to the rescue.

Anyway, thanks to Slowy for allowing me access to the guts of this little beast. I don't think he is missing it much at the moment, seem to have fun with something burgundy...

Necessary safety improvements I would like to make:
a - Put a fuse in the mains supply as a minimum
b - Place a 10nF capacitor in the input circuit on V1 to decouple the guitarist from high voltages in the unlikely event that V1 grid shorts to anode.

Observations:
- Input circuit very much Fender early days
- Tone stack simple but very effective, ie, works well
- No resistors and caps in the power section, other than the biasing.

Questions:
1 - What likely value is C5? It is a pipe capacitor but no value is shown. My guess would be about 1nF or so seeing it is part of the treble filter.
2 - R17 and R18 are in series. Why use two resistors and not a single one? Also these two resistors measure 390K each in circuit!!! That is a hell of a change.
3 - What should the wattage be for the HT supply voltages (R20, R21, R22)?
4 - I would like to change the rectifier section to a full bridge one to allow for a smaller transformer. Good idea? Impact on tone?
5 - What is a ECC803S? Equivalents?
6 - Anything wrong or unexpected with the circuit diagram?

I hope the well respected amp builders on this forum will chip in so I can finalise the circuit diagram in preparation for building one.

The resistor values are as per colour bands, not measured values.

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User avatar
BY GrantB
#807568
First up, excellent work. I am in the “keen to replicate” camp.

I’m happy to discuss with our local amp royalty Mr. Gilmore if you’d like. See if he can assist.
User avatar
BY Slowy
#807578
Thanks Jvpp. I have no technical knowledge but can fill in some provenance.

It was built in Auckland in 1966. The person I acquired it from bought it in 1988. It can be heard as the bass amp on all Lucid3 albums and was used for guitar overdubs on some of their tracks as well. I found it in the dusty corner of a studio 10 years ago and sent it directly to Clarry Schollum who is the only person who has been inside it since the 80's. He did indeed replace a cap and some resistors; also added an impedance selector and pronounced it, "good for another 50 years." He measured output at 12 watts clean and 18 watts distorted.

ECC803 is a long plate JJ 12AX7. IRIC, I have a 5751 in V2. I liked the slightly increased headroom it gave. With a 12AX7 in there, there's much earlier breakup.

I think it's a great little amp. If there's anything I can do to support this project, I'd be happy to.

One final point: Don't think for a second that I don't miss it! :lol:
User avatar
BY StrummersOfThunder
#807580
maybe we should mock up some turret board? Ive got a bunch of garolite. Just need to come up with a sensible layout. Then whom ever wants to have a crack at building one can just paint by numbers
User avatar
BY jvpp
#807581
GrantB wrote:First up, excellent work. I am in the “keen to replicate” camp.

I’m happy to discuss with our local amp royalty Mr. Gilmore if you’d like. See if he can assist.


That would be most welcome :D
User avatar
BY jvpp
#807582
StrummersOfThunder wrote:maybe we should mock up some turret board? Ive got a bunch of garolite. Just need to come up with a sensible layout. Then whom ever wants to have a crack at building one can just paint by numbers


Yes that would be nice. May take you up on that once final circuit diagram has been drawn...
User avatar
BY jeremyb
#807583
Watching with interest!! What exactly is it about the characteristics of this amp that has everyone fizzing? EL84's and not an AC15 style of circuit?
User avatar
BY Slowy
#807590
jvpp wrote:The ECC803 is V2. 12AY7 in V1.

Thank you. I stand corrected.
I have a box full of valves that have been through this amp. Stopped when I was happy and haven’t looked for a while.
Just tell me you don’t have my NOS Amperex and Mullards in there.
User avatar
BY Conway
#807591
jvpp wrote:Place a 10nF capacitor in the input circuit on V1 to decouple the guitarist from high voltages

This might be why you often see smoke rising from Slowy's head when he's playing.
User avatar
BY sizzlingbadger
#807597
b) Not many amps have a safety cap on the input but its not a bad idea, keep it large enough to not lose low end
1) Probably 250pF
2) The joint at R17 & R18 will go to the bias circuit, it shouldn't just be floating.
3) 5-10W would be fine, can't really tell without knowing the voltage drops
4) might have less sag, maybe, but nothing to worry about tone wise I would think
5) ECC803 is just high quality version of an ECC83/12AX7 - JJ still make them today
6) I'm surprised they didn't use the second half of V1 to create a cathode follower to drive the tone stack - but hey :-)
Check R1, I would use 1M to make sure the grid leaks properly.
Last edited by sizzlingbadger on Fri Nov 09, 2018 8:39 am, edited 1 time in total.
User avatar
BY Optical
#807598
jvpp wrote:Questions:
1 - What likely value is C5? It is a pipe capacitor but no value is shown. My guess would be about 1nF or so seeing it is part of the treble filter.
2 - R17 and R18 are in series. Why use two resistors and not a single one? Also these two resistors measure 390K each in circuit!!! That is a hell of a change.
3 - What should the wattage be for the HT supply voltages (R20, R21, R22)?
4 - I would like to change the rectifier section to a full bridge one to allow for a smaller transformer. Good idea? Impact on tone?
5 - What is a ECC803S? Equivalents?
6 - Anything wrong or unexpected with the circuit diagram?


Good work

1. The tone stack looks very close to a Vox design. C5 would be a low value, 47p in the Vox. However R7 doesnt look right, 1M5 would make the tone controls do almost nothing - check the code again as it might be 15k. The Vox value is 10k for example.

2. R17 and R18 should have a ground connection at their junction. These are the standard grid leak resistors for a cathode biased power amp.

3. Anything that wont smoke :) 5+W will do.

4. Why? wont do much or anything...

5. just a 12ax7
Last edited by Optical on Fri Nov 09, 2018 8:46 am, edited 1 time in total.
User avatar
BY sizzlingbadger
#807600
R7 & C6 Good catch

2) agreed, these should go to ground, its a cathode biased, sorry was reading it at fast during work !

That V1 is a bit strange having the cathodes linked but only using one side, maybe just the way they wired all the sockets.
User avatar
BY GrantB
#807601
I made some toast this morning.
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