Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bullkelp on September 14, 2025, 01:39:36 pm
-
Hi All
Im in the testing stages of this build. Its faithful to the mojotone schematic other than increasing the vaalue of the 3w Bias setting resistor to 2k rather than 1k.
I had used some parts from an old deluxe reverb build (reverb driver transformer, choke, pan) but the PT and OT are new hammond iron (PT 290cex and OT 1760j)
Plate voltages 430
Plate current 42mA
Im yet to go through the rest of the voltages. I will do this tonight.
Clean sounds fine, the trem works well. The reverb is meh... but ok
The drive/distortion is splatty and almost blocking type.
Questions:
1. there is a 470r vs 4.7k NFB tail resistor discrepancy between schematic and layout from fender OG paper work. Im right to opt from 470R (along with the 10k NFB) ?
2. Should I use grid stopers (1.5k) like the aa864 etc?
3. The 63 reissue uses a 100pf cap across the plates of the PI. Is this reccommended?
4. The 6g16 calls for a 12ax7 in the reverb driver, where as the deluxe reverb uses 12aT7. Which is the right way to go here?
5. Im using the 8ohm tap of the 1760j as the cab will have an individual 8ohm 12" driver. Would the original have used the 4ohm tap given it was a 2x10? Do I there fore need to make any adjustments to the NFB resistor values?
Many thanks
-
Questions:
1. there is a 470r vs 4.7k NFB tail resistor discrepancy between schematic and layout from fender OG paper work. Im right to opt from 470R (along with the 10k NFB) ?
2. Should I use grid stopers (1.5k) like the aa864 etc?
3. The 63 reissue uses a 100pf cap across the plates of the PI. Is this reccommended?
4. The 6g16 calls for a 12ax7 in the reverb driver, where as the deluxe reverb uses 12aT7. Which is the right way to go here?
5. Im using the 8ohm tap of the 1760j as the cab will have an individual 8ohm 12" driver. Would the original have used the 4ohm tap given it was a 2x10? Do I there fore need to make any adjustments to the NFB resistor values?
Many thanks
My 2CW
1. If using a 12AX7 in LTP keep the bias resistor at 820R. Note the Fender tail resistor is 6k8. Otherwise use a AB763 LTP with a 12AT7
2. No harm. You can go up to 47k for the output tubes without losing any highs
3. See how you go once you light it up
4. I prefer a AB763 reverb
5. YMMV. NFB can make the amp tighter or rawer. Nothing is ‘bad’ if you like it.
-
Ok ! I think we’re good
I didn’t grid stoppers and NFB
I also found a tiny little mistake in the PI ….
For some reason I’d grounded pin 6 :laugh:
She sounds marvellous
Thanks again
-
Quick little test from the garage last night
-
Quick little test from the garage
Sounds good - 2x10s?
-
Quick little test from the garage
Sounds good - 2x10s?
thanks
Thats just through a little test cab i keep in the garage. its and open back 1x12
-
Sounds lovely, especially with the tremolo.
-
Here is another quick demo.
This time with p90s.
Really gets that edge of break up tone !
Stoked.
-
Quick gut shot
-
Heres another quick demo with my JM
Also a tiny bit of halcyon klon transparent boost on the JM
Interesting ohase effect of the vibrato on the loop competing with the lead vibrato (particuarly with those dissonant chords)
-
Well all was going well with this.
Spent today tolexing the cabinet even.
But this afternoon I put the amp up the bench to try some tweaks with the tremolo. I didnt get very far (had tack soldered 0.01uf cap to one of the 0.01uf caps in the tremolo oscillator to try and slow it slightly). Didnt have much effect so I removed it, seemingly leaving the amp as it was.
When I turned the amp back up the fuse went immediately. I replaced the fuse and plugged into the current limiter. The fuse that was in there was 500ma as it had come from a deluxe reverb. Ive put in the only one I have to hand, which is 3A. I think the 6g16 calls for 2A?
Anyhow its functional again but I note some heating of my current limiter bulb...
I wonder what sort of short/issue ive caused?
Appreciate the advice
-
You were running an undersized fuse. A slight glow to the bulb is expected, especially if you're using a 60w or smaller bulb.
A bright glow indicates a short. You can always stick an ammeter in line to confirm.
6G16 schematic shows a 2A fuse. These were typically the delay (slow blow) type.
And for slowing down the trem, I find I get better results by also increasing the cathode capacitor (at least 50μF, but usually 250μF) at the same time as doubling one of the oscillator caps.
-
A 1A slo-blo mains fuse is quite sufficient with 230 vac at the wall. 500mA might be inviting blowing- especially with the silicon grain oriented steel in modern PT laminations.
Try tacking 0.02 in parallel with one of the 0.01s in the LFO
-
Well I changed to a 2A slow blow and all seems well.
I found an. issue with the second preamp tube down stream from the ouput tubes. It would intermittently short if tapped. Changed it and things seem cool.
I cant detect any other shorting and everything is functioning.
This is a cool amp. I love the warm smooth tones it gives on the neck pickup of the JM and the crunchy setzer tones on offer witha bit more digging, particularly on the bridge.
This clip is via a creamback 65.
-
That’s a dream combination. Awesome