1969 Pro Reverb -

[Mr Pro Fender]

the heater Vac is 3.0 VAC (on pin 7 and 2)

Transformer Bigone - 68249

Choke - 606-9-10 (022699)

Output - 606-9-10 (022848

6L6 plate dissipation is 20 in one 6L6 and 15 on the other this is low but I have at these at 30 and still distorting around low volume.

[pdf64]

Interesting that there's no manufacturer or Fender part # code on the power transformer, just a date code?
Any yellow leads tucked away in there (stock PT would have them, 5Vac heater supply for tube rectifier)?
Regards regular heaters, it's best to measure heater voltage across the heaters rather than referenced to ground, ie between pins 7 and 2. I know it ought to add up and amount to the same but it can end up a little different.
6.0V is low for the heaters though the VB+ seems higher than expected, even more so than the silicon diodes would be expected to cause (I had been wondering if the mains/line voltage selector had been set correctly, they often end up on 220 or 230V).
Power tubes on fixed bias are usually biased so as to be idling somewhere in the range 50-70% of the max plate dissipation, so 15 -20 watts is reasonable; 30 watts would be over stressing the tube and greatly shorten its useful life, as well as stressing the power supply.
Where to go from here?
A complete Vdc measurement of every tube plate/grid/cathode may reveal something.
Check of every resistor value (especially 6L6 470 ohm screen grid resistors).
A cause of mushy bass and lowish power is often failing B+ caps. The Vac riding on the B+ Vdc, at no signal and full power, may indicate something.
Additionally, Pros have an undersized output transformer, you can't expect the same bottom end at high power that a super reverb would provide.
The Aiken site linked previously and Geofex Low Power or Loss of Volume are fantastic resources.
Pete.

[Mr Pro Fender]

I know the screen grid resistors have drifted a little around 463 and 476 these are the two readings
Both plates are 519 vdc on bo pin 3 of both 6L6s. Screen grid is 519 again on both (pin 4).

Few things here I dont know how to test:
1: measuring cathode vdc?
2: The Vac riding on the B+ Vdc, at no signal and full power

Pros have an undersized output transformer - could I put in a bigger output to give more headroom if so what would you recommend here?

what i'm after is making the amp as clean as possible at high volume with a good clean bass - I use a 058 as my bottom E string trying to keep it from going mushy is tough.

[pdf64]

When checking resistors, the key things are
a/ is the value in spec, eg 470 +/- 5% or 10%; yours are fine
b/ any suspicion of damage/overheating/physical degredation. Yours are chunky and appear fine on the pics. Generally if in doubt, clip on the meter leads to the resistor and waggle the resistor around a little; if the lead terminals are loose etc in the body then the resistance will change significantly (maybe just for a moment) or may even go open circuit. As mentioned, I would do the above with every resistor in the amp; it's often far more efficient (and more reliable for the future) to replace them all than to do this. However, for your peace of mind it's nice to identify the faulty part; the main suspects are screen grids, power supply B+ droppers, plate and cathode resistors; basically any that pass significant dc, particularly so if they get hot in normal operation.

1/ just measure the voltage between ground/chassis and each tube cathode (exclude the 6L6 as they're direct to ground at the moment), so terminals 3 and 8 of each 12A?7. Same for grids, terminals 2 & 7.
2/ Set meter to Vac and connect probes to ground and B+ (eg at rectifier output/output transformer centre tap). Expect 5-10V at no signal', 10-20V when amp is cranked.

Output transformer - Philip at Bluebell sells the Hammond range, which are good and reasonably priced. Depending on what output impedance options you want, the 1760K, 1750L, 1760L or 1750M would be suitable Guitar-trans-output

As you B+ is so high, it's very likely that your amp should be putting out a higher power than the 022848 was intended to cope with, resulting in magnetic saturation on low notes at high power level, which is probably a bad thing from your perspective.

Just a note regards safety; make sure to use an RCD circuit breaker (same as for electric lawnmower) when working on open live amps and tape off any exposed mains or B+ terminals that you may accidentally touch. In regards of stopping your heart, B+ is propably a more significant hazard than even 240Vac mains.
Pete.

[Mr Pro Fender]

Thanks Pete I'll get to checking these out and ill post back later today - RCD circuit breaker all ok thanks.

2/ Set meter to Vac and connect probes to ground and B+ (eg at rectifier output/output transformer centre tap). Expect 5-10V at no signal', 10-20V when amp is cranked.

does the speaker need to be plugged in for this test?

and with output transformers what would a 100watt for the fender Twin do if put in a 40 watt pro reverb I can see primary ohms are different (4,200 C.T for the Bassman the 1750L push pull - 50 Watt) to (2,000 C.T for the twin which is 1750W push pull 100 watts).

Would this give more head room? being bigger in watts?

[pdf64]

Yes, always ensure that an operational tube amp has a suitable load.
The OT from a twin reverb would be ok if you could have an 8 ohm load on it.
However, the pro cab speakers can only be wired for a 4 or 16 ohm load and with such a load mismatch, the 'pro with TR OT' would put out less power (about 30% reduction as a rule of thumb).
So I don't advise doing that.
Pete.

[Mr Pro Fender]

I found from that site you gave a 1760W which has ALTERNATIVE 4, 8, 16 push pull 2.000 C.T - 100 watt for a Dual Showman and Twin Reverb - so this should work with the 8ohm tap.

I thought the pro was 4 ohm as its a 2X12 same as the twin I have. the reading are the same to from the secondary of OT.

[Jazz P Bass]

:
what i'm after is making the amp as clean as possible at high volume with a good clean bass - I use a 058 as my bottom E string trying to keep it from going mushy is tough.

Just my 2 cents.
I feel that you are barking up the wrong tree.
A 40 watt amp is not what you need.
The Fender Twin amps are able to go flat out without breaking up. (with a little preamp tube tweaking)
That is what you want.
Even then you may not keepupwith a lousd drummer.
I would get a solid state amp.
A Hartke or something similar,that is designed for punchy bass response.

[pdf64]

The OT acts like a gearbox, so to get the 1760W to provide a 4k load at the primary, would need to put an 8 ohm load to the 4 ohm secondary tap, ie double how all the secondary taps were labelled.
Yes the pro will be 4 ohm, ie 4 ohm speaker load (2 x 8 ohms in parallel) and an OT intended for a 4 ohm load, to provide 4k at the primary.
The speakers however could be wired to be 16 ohms.
Anyway, I don't think that the 1760W would bring any benefits, and there's potential for confusion over which tap to use - you would have to wire the speakers to 16 ohms and connect to the 8ohm tap, in order to provide 4k at the primary.
The 50 watt types are plenty beefy enough, spec'ced for full power down to 70Hz, ie you could down tune to C#.
Pete.

[Mr Pro Fender]

thanks for sharing - like ive put these are things ive picked up over the years of gigging and working with other top players. The Twin is great by way to loud for smaller clubs the pro is a great amp I think it just needs love a few mods and some parts replaced. The distortion I have now is the bad kind at the wrong level ( around 4) I have used this amp on 8 and done almost all the set with a great tone - then it got mushy.

[Mr Pro Fender]

Here is a live recording with just this amp at a small club have a listen
Rock Steady with Horns Live by tommyallenband on SoundCloud - Create, record and share your sounds for free

this was on around 8 with volume. Cant get the amp to that level now and sound good? I'd still like to get the bass clearer (bassy but clear) a good full tone (This is 2X12) maybe a 1X 15? anyway enjoy the track and let us know ant thoughts.

[Mr Pro Fender]

Hi pete, with kids in bed ive had space to do them tests - 1/ just measure the voltage between ground/chassis and each tube cathode (exclude the 6L6 as they're direct to ground at the moment), so terminals 3 and 8 of each 12A?7. Same for grids, terminals 2 & 7.

Starting at V2 - Pin 3 (002) Pin 8 (001.5) Pin 2 (0) and Pin 7 (0) (Pin 2 goes off to the input jack channel 2)
V3 - Pin 3 (009) Pin 8 (009) Pin 2 (0) and Pin 7 (0)
V4 - Pin 3 (002) Pin 8 (002) Pin 2 (0) and Pin 7 (0)
V5 - Pin 3 (0) Pin 8 (0) Pin 2 (049) Pin 7 (049)
V6 - Pin 3 (124) Pin 8 (124) Pin 2 (078) and Pin 7 (080)

I tried 2nd test 2/ Set meter to Vac and connect probes to ground and B+ (eg at rectifier output/output transformer centre tap). Expect 5-10V at no signal', 10-20V when amp is cranked.
SO I set to AC Volts black to chassis and red to centre tap of output transformer and I get 1140 VAC???? so unsure if i'm doing this test correct?

Ive taken some more pics so ill past them up.

[pdf64]

Hmm, nothing odd yet (I assume V1 slot is empty, hence V2 pin 8 is a bit low) - could you check the plates too, terminals 1 and 6?
Some meters won't measure Vac on top of a large Vdc, it looks like yours is one of them; I know most Flukes (eg 73 MkIII) I've used will cope. Can anyone recommend something else (cheaper!) that will do this measurement?
BTW, I've not been able to get that soundcloud page to load yet?
Pete.

[Mr Pro Fender]

Here are the plate readings

V2 Pin1 (229) Pin 6 (188)
V3 Pin1 (514) Pin 6 (514)
V4 Pin1 (261) Pin 6 (250)
V5 Pin1 (509) Pin 6 (452)
V6 Pin1 (301) Pin 6 (311)

My meter wont do that test then as it's reading too high - is there another way to do the test.
Ill have a look at the sound clip it should work as I have it up and playing? does the clip come up at all or is the whole screen not loading?

[pdf64]

OK, nothing out of the ordinary.
Running out of ideas now.
Something has changed to reduce the sound level from the amp and quite a lot of things have been eliminated.
We're left with the power supply caps as being a key suspect that hasn't been checked yet.
Is the wiring to and from the power supply caps ok? Good solid solder joints?
Have you got any spare caps you could sub in? If not new caps, then known good ones from another amp.
Another possibility is that a signal coupling cap has gone high resistance or open circuit, but to trace that out you'd need a sig gen (or anything that can generate a constant tone) and a dummy load (preferably).

The whole soundcloud site seems to be down, get a 'Internet Explorer cannot display the webpage' on everything.
Pete.