Bassman 50 not loud enough - what could it be?

[spinfresh]

I got a very rusty bassman 50 for a super good deal since it was all original I figured what could go wrong?

well so far I am baffled

so far I have replaced filter caps and resistors - 2 of the cathode by pass caps and resistors - put in a new volume pot and treble pot on the bass channel, replaced the tubes with New jj's re-tensioned and cleaned all of the tube sockets - replaced the 470 ohm resistors on the 6l6 tubes.

while it now works and the crackleing has stopped it still is not very loud. even turned up to 10 I get breakup but very little volume.

I have connected my scope to the output of the phase splitter (with a 1khz tone) and I get a very nice 180 degree out of phase wave - I connect my scope to the blue and brown wires of the OT and I get a very nice saw tooth wave that looks the same for both tubes (2 ch scope)

the voltages appear correct - my input mains is 120v spot on set with variac
the voltages in the amp are very close to the schematic

any ideas? thanks in advance

P~

[Jazz P Bass]

What is the input voltage of the signal generator? (I use 100mv's)

What is the output voltage at the load?

What is the load?

[DrGonz78]

Since it is easy, perhaps try swapping some of the preamp tubes just to make sure there is not a weak one in the mix. I am betting that you replaced the pots with the correct value but just always double check.

[Leo_Gnardo]

I connect my scope to the blue and brown wires of the OT and I get a very nice saw tooth wave that looks the same for both tubes (2 ch scope)

Is that sawtooth at the test frequency - and large in amplitude? or 120 Hz and just a couple volts riding atop the expected 450 or so B+? If 120, it's not unusual to see some remnant of not-so-well filtered DC at the output tube plates. OTOH if it's the test frequency, I'm thinking first lets have a look at the output tube cathode circuitry & make sure everything's what it should be there. Because I'm suspecting maybe a bad OT. Which would stink, but it does occasionally happen. If everything looks OK at the 6L6's cathodes... my best method for testing the bad OT is sub in whatever known good OT you may have on hand, and if good looking output results then the original is toast.

You don't have to go whole hog on the sub, just unsolder the original OT's 5 leads, and use clip leads to temporarily sub in your test OT.

[g1]

Also, if you don't have a high voltage probe, scoping signal at the power tube plates is not very safe.

[nosaj]

Also, if you don't have a high voltage probe, scoping signal at the power tube plates is not very safe.

I use a .001 600v cap attached to my scope probe with the other end of the cap has a hook.
nosaj

[spinfresh]

Is that sawtooth at the test frequency - and large in amplitude? or 120 Hz and just a couple volts riding atop the expected 450 or so B+? If 120, it's not unusual to see some remnant of not-so-well filtered DC at the output tube plates. OTOH if it's the test frequency, I'm thinking first lets have a look at the output tube cathode circuitry & make sure everything's what it should be there. Because I'm suspecting maybe a bad OT. Which would stink, but it does occasionally happen. If everything looks OK at the 6L6's cathodes... my best method for testing the bad OT is sub in whatever known good OT you may have on hand, and if good looking output results then the original is toast.

You don't have to go whole hog on the sub, just unsolder the original OT's 5 leads, and use clip leads to temporarily sub in your test OT.

Well... you are correct - the transformer is bad... thank you all for the responses... I tacked in a known good transformer and bam nice and loud.

[Helmholtz]

I use a .001 600v cap attached to my scope probe with the other end of the cap has a hook.
nosaj

The capacitor only blocks DC. It is essentially non-existent for AC voltages. In particular it won't prevent arcing and damage within the probe caused by AC voltages in the kV range in case of HF oscillation.

[J M Fahey]

And 600V DC is not nearly enough

With, say, 450V +B , each plate swings from essentially ground to twice +B , so 900V peaks ... WAY over cap rating.
FWIW I use a .01 1600V DC one there

And thatīs with a nice sinewave and resistive load, as soon as amp squarewaves and load is inductive (as in *any* speaker ) you can easily have 1400V peaks.
Or worse if speaker/load got disconnected.

Same principle is used in those SMPS which accept any input from 90V to 260V mains without flinching, and car ignition, go figure.

Where they get 10X the peak voltage they would based on turns ratio alone.

I guess our friend spinfresh didnīt fry his scope (or himself) *just* because the shorted turn transformer could not kick back.

[nosaj]

The capacitor only blocks DC. It is essentially non-existent for AC voltages. In particular it won't prevent arcing and damage within the probe caused by AC voltages in the kV range in case of HF oscillation.

In that case a standard AM portable radio should determine if your oscillating. General Voltage checks can also detemine if HF oscillation is sucking power.I just use the cap for general checking with my scope probe.
nosaj

[Helmholtz]

I use a .001 600v cap attached to my scope probe with the other end of the cap has a hook.

From the context this sounded like an advice to increase probe safety regarding HV measurements (especially at power tube plates).
I said it doesn't help much with AC safety, as AC voltage before and after the cap are the same.
As Juan explained, peak voltages at the plates can get as high as 2.5 times HT even without oscillation. Probing the plates may induce high oscillation voltages even without signal.
Blocking DC with a sufficiently rated series cap helps somewhat, as the peak voltage rating of probes is the sum of DC and AC peak, but it will never turn a 500Vrms probe into a 1.5kVrms one.