5F6-A Build Stumped. Thank you!!!

[JimmyJam]

Hi All! New to the forum, been repairing tube amps, mostly Hammond organs and leslies, but I also repair guitar amps.

I'm stumped on this one.

I've been building a 5F6-A circuit for use with an 8 ohm output load out of an old Bassman Ten that I gutted. The only parts that remain from the bassman ten is the chassis, some pots that tested good, jacks that have all been cleaned, etc. the power transformer and eyelet board.

Most internal components are new, except for a couple resistors in the preamp that were still in spec, including new tube sockets for the rectifier, power tubes, and PI tube.

I'm using a Heyboer choke designed for a tweed bassman, and a new Heyboer output transformer originally designed for Marshalls, but it's a 3.9Kohm primary CT (tweed bassman uses 4k primary), with an 8 ohm secondary tap (tweed bassman uses 2ohm secondary). I'm using a brand new Fat Jimmy 15" C1570, that sounded great when hooked up to another amp. 

I have my brother's flawless '59 RI on the bench that I've been using to help troubleshoot for bugs, that sounds amazing.

Here's my problem. I have low volume distortion, starting around 2-3 on the volume control, and gets really bad as I turn up the volume. Presence also makes it worse. In addition, my output signal is about 20% weaker than when I run the same strength input signal into the '59 RI.

I have the B+ voltages matched using a variac, and all point voltages and pin voltages are within several volts of each other when I compare the two amplifiers, and within spec (at idle) as compared to the RI schematic.

Bias is set to 35mA on both amplifiers.

I have swapped the full tube sets between the amps and the symptoms remain unchanged with my build.

Using a scope, and a signal generator, I applied a signal to both amps, and adjusted the volumes to provide the same amplitude signal into the PI at both amps (volume controls were almost the same, all other knobs all the way down). The signals look identical between the two amps coming out of the phase inverter, and going into the power tubes.

HOWEVER, leaving the power tubes going into the output transformer, the amp I'm building has a peak to peak signal approximately 20% less in amplitude with noticeable clipping, whereas the '59RI has a clean signal with greater amplitude.

Where should I look next? What should I be considering?

If you got this far, Many thanks!!!!!! I hope we can solve this together :)

Cheers!! - Jim

[shooter]

leaving the power tubes going into the output transformer,

a quick n dirty, use a dummyload for your speaker n verify the same as ^^^^^
or swap speakers with good amp


if your load is bad it can reflect back to the primary.

[JimmyJam]

Hi Shooter, Thanks for the reply. I have tried the speaker in other amps, where it sounds great, and hooked up another 8 ohm load to this amp and have had the same symptoms.

[stratomaster]

Try temporarily disconnecting the negative feedback from the speaker and see if that resolves it.

If not you can run the preamp of the good amp into the PI of the bad amp and vice versa with alligator clips to quickly find where the problem lies.

[tubeswell]

You say 'voltages are within spec' but you haven't told us what the voltages are. Measure and report back on all plate and cathode voltages (including preamp) and all filter cap power supply node voltages. (If a filter cap in the preamp isn't electrically connected to ground, it will cause signal feedback between gain stages through the power supply rail, which causes mushy spongy distortion).

[JimmyJam]

Boy oh boy…. I learned a good one. So I’ve been using a bias probe to make bias adjustments more “easily”. Easy until you leave them both in. The tool only completes the circuit for tubes that are taking readings meaning the other tube is essentially disconnected. Once I realized that, and plugged both tubes in directly… it sounds absolutely killer!! Blows the ‘59 reissue out of the park!! Sometimes it’s something so simple! Thanks everyone!!!

[mresistor]

You could install 1 ohm 1 watt 1% resistors on the cathodes of the power tubes (between the cathode and ground) and then just measure the millivolts across the resistor which translates directly to reading of milliamps of total tube current.  Then you can subtract a few millivolts to compensate for the screen current. It's a much easier way to set bias because all you need is a meter, although two meters works better as you can see plate voltage and total tube current at the same time.


Folks here also recommend just 1/2 watt resistors which will work fine as well. 

[mresistor]

One could install a safety diode in parallel with the 1 ohm resistor,, but  I don't like that personally because I want the 1 ohm resistor to blow in the event of a catastrophic failure of the tube.