5F6A Bassman total custom build - super low output

[tomgilmartin]

First build. First off, let me save you some typing. I know you all have safety in mind so: DISCLAIMER - I know about lethal voltages, one hand for testing, etc. If you still have to tell me, no problem. In case anyone cares, I've been an electrical engineer for 25 years, currently licensed in 14 US states and 2 Canadian provinces. That said, I still have a lot to learn about tube amps.

I built a Bassman like you have never seen. 6 months of research and fabrication. I have been on Robinette's site almost daily. You might ask, "Why would anyone build something so elaborate?". No other reason than because I can, and I want the BEST tone.

Amp Specs:

• Custom chassis in 5" aluminum tubing.
• Teflon tube sockets
• Components direct-wired to the sockets wherever possible.
• Star grounding at V1, V2 and V4/V5
• Isolated input and output jacks (not grounded at jack).
• Grounds connected in twisted pairs with corresponding signal.
• All wires to pots and switches twisted pair with corresponding ground.
• Tubes are preferred series from thetubestore.com.
• Cermet potentiometers throughout.
• Wired for 12VAC heaters on V1, V2, V3.
• Edcor USA made transformers. (These are gorgeous!)
• Fused input xfmr secondary
• 8ohm, 50W power soak resistor for crunch at reasonable volume!
• All resistors 1%, and 1W or more except where unavailable.
• Wired with 20AWG solid wire from McMaster Carr in wacky color scheme.

Speaker cabinet:

• Custom cherry/maple/walnut tubular cabinet.
• (2) Acoustic Elegance Dipole 10 speakers. (aespeakers.com)
• Wired with military grade 16AWG teflon twisted pair cable.
• 1/4 male plug

Mods on this amp from Rob Robinette and others:

1. Boost mod - bypass V2A cathode resistor with 0.68uF cap.
2. Tone stack mod - Opens ground to bypass tone stack.
3. Humdinger 500ohm pot between the 2 power tubes.
4. Adjustable bias
5. Adjustable feedback (50K pot, 13K min resistor in series)

Unfortunately, it doesn't make much noise. With a guitar in any input and hard strumming, the output is barely audible. Clean to half volume, then scratchy to max volume. On the normal inputs, it squeals just a bit at max volume. It gets just a bit louder if the bias is turned hot. I only did that for a few minutes.

Voltages:
V1 PRE 12AY7
1: 233
2: 0
3: 2
4:13VAC
5: 13VAC
6: 245
7: 0
8: 2
9: No Conn

V2 GAIN-CF 12AX7
1: 147
2: 0
3: 1
4: 13VAC
5: 13VAC
6: 341
7: 147
8: 149
9: No Conn

V3 PH-INV 12AX7
1: 272
2: 14
3: 46
4: 13VAC
5: 13VAC
6: 275
7: 16
8: 46
9: No Conn

V4/V5 POWER 6L6GC
1: No Conn
2: 6.4VAC
3: 464
4: 462
5: -51
6: No Conn
7: 6.4VAC
8: 0 (Grounded)

Bias -52VDC (Cool on purpose for now).

OT rated for 3.4K input, 4ohm output
Disconnected OT leads from output jack:

• Output to parallel 8ohm resistor and speaker cab measures 5ohms.
• OT output leads measure 2.5ohms.
• Impedance of OT input side is about 80ohms to center, both sides

Connected an mp3 player to treble pot wiper, could barely hear it with player at full volume.

Any ideas why no real output?

[Randall]

I would run a tone thru it and trace the signal with a scope to see where it is getting lost.

[The Dude]

Welcome to the place!
Tube voltages look reasonable at a glance. The thing that jumps out at me most is your secondary OT resistance. 2.5 ohms seems a bit high. Have you checked your meter zero? What does the meter say if you just touch the leads together? Also, your primary readings of 80 ohms outer to center are resistance and not impedance? Do you have a scope? I would also probably ditch the power soak resistor until you get the amp working.

[tomgilmartin]

The Dude
Welcome to the place!
Tube voltages look reasonable at a glance. The thing that jumps out at me most is your secondary OT resistance. 2.5 ohms seems a bit high.
--> Right, my meter is a cheapo. I have a Fluke at work, but I ain't dropping $900 for my own!
Have you checked your meter zero? What does the meter say if you just touch the leads together?
--> Leads touched it says 0.9ohm. I know it's a POS.
Also, your primary readings of 80 ohms outer to center are resistance and not impedance?
--> Yes just cheap meter resistance.
Do you have a scope?
--> No. Maybe I could borrow one.
I would also probably ditch the power soak resistor until you get the amp working.
--> Funny, I cut the lead just one minute before your reply. It's louder, bur still not even CLOSE to where it should be. I like it because when it's connected there is no way to turn on the amp with no load.

[The Dude]

Your on the right track injecting signal directly into the PI. You've narrowed the problem down quite a bit already, but it would be nice to have a scope to see what happens after that. I'd try injecting a test tone at that same spot and see if your meter will let you measure levels using AC volts since you don't have a scope. You can get MP3 test tones all over the web to put on your player. With signal applied to the PI grids, it would help to see what's on the output tube grids and what the level is.

[Randall]

Good Lord, after seeing those photos I am at a loss. I will sit this one out. By going so far out of your way to break time tested layout convention, it may well be wired exactly, but have parasitic and/or blocking distortion issues. Hook a scope up and see what's actually happening that maybe you can't hear.

[The Dude]

Yep. The pics do little to help in this case.

[Justin Thomas]

Would a humdinger pot be 500k? Didn't we just say on a different thread that said his measured 0-30k that even THAT was grossly out of whack?

Justin

Checked Rob's site - specs a 100-250R linear pot.

[The Dude]

I think "humdinger" is something different in this thread?????? He says "Humdinger 500K pot between the 2 power tubes". If we're talking about the same other thread, I believe that was a hum balance control, which would be across the filament line as an artificial center tap.

That does bring up a point, though. Can we get a schematic of what actually was built here? It would help to know for sure what we're dealing with.

[Chuck H]

Ok... I very almost posted the roflmfao guy, but that would have been rude. My reason, though, was because there was so much effort put into the project in areas that actually DON'T have much to do with amplifier tone and I think this may have resulted in a parasitic oscillation that's eating up watts that should otherwise be available for amplification. Layout is critical for guitar amplifiers because they are very high gain devices. The build clearly had to ignore that to achieve it's somewhat obscure (at least to me) goals. So, just shy of snarky, I know. But I still hope to be helpful and the build effort is so pretty it would be a shame not to try. So, I think it would be a good idea to test the amp for current draw under no signal and low signal operating conditions. If you find that the amp is drawing significant current it may be that there is an ultrasonic oscillation squandering power. That is, the amp is working very hard amplifying something arbitrary you can't hear.

[Justin Thomas]

I just caught the "...And others..." So, Hard to tell where it came from. Never heard of such a thing.

Justin

[Randall]

^^^
Too cryptic for me too understand.

[Justin Thomas]

^^^
Too cryptic for me too understand.

The first post said he included the following mods, from Rob Robinette "and others." I missed "and others" and was trying to say, 1- no idea where a 500k pot between tubes mod came from, and 2) it's not on Rob's site that I could find.

Justin

[Pedro Vecino]

Is the secondary of the output transformer connected to ground?

[tomgilmartin]

Is the secondary of the output transformer connected to ground?

Yes, at the feedback connection point near the presence pot.