1969 Pro Reverb -

[Mr Pro Fender]

I’ve picked up a 1969 Pro reverb AA165 Silver Face.

Now when the guitars plugged in it distorts when turned above 4 on the Volume.

I have 54VAC going to Bias board then a 1K resistor then diode after that I’m reading (-66 DC) I then have a 32uf 64 V cap. From here to bias pot then to the two 220K resistors - at pin 5 of the 6l6s I’m getting ( - 58).
I’ve tried different speakers and different 6L6s.

Distortion still there

I’ve tried a different out put transformer - but still does it.
The out put transformer with the pro reads
Primary to centre tap 1st 6L6 is 73.6 ohms 2nd 5L6 is 82.5 ohms
and the secondary reads 0.3 ohms

So I’m really stuck where else to look for this distortion when you get past 4 on the volume pot.

Can anyone help point us in the right direction?

[pdf64]

Great amp but bear in mind that you have an amp full of 40+ year old components; the resistors and electrolytic caps are best regarded as consumables (they will age and few are likely to still be in spec) after that length of service and so be replaced.

What guitar are you using, as in how hot are the pickups? My les paul is way louder than my strat and I'd expect it to be getting crunchy past '4'.
What voltage is it putting out to the speakers? When it just starts to break up and at full kerrang?
See schematic http://www.webphix.com/schematic%20h...a165_schem.pdf
Suggest you compare the Vdc on your amp to those of the schematic and report back.
Pete.

[Jazz P Bass]

An awful lot of Fender amps start distorting at 4 on the volume.
You really do need a scope & a signal generator to know how much & when.

[Mr Pro Fender]

Playing a 90s standard strat. its hard to judge the voltage i'm using a multi meter on AC clipped to speakers (is there a better way? without a scope) and volume on 4 depending how hard I hit the strings I get between 11.0 vac to 4.0 vac (avg 6.0 vac). sounds distorted mushy - loud though.

[Mr Pro Fender]

I live in Wales so unsure where I could get it tested on a scope round here? - i'll put my thinking cap on. there's no one this way who fixes vintage valve amps.

[Jazz P Bass]

Does the amp sound o/k at 2 or 3?
This is a 40 watt amp.
Where the clean output ends & where the distortion starts is the deciding factor.

[pdf64]

Both channels the same?
11 Vac isn't too bad.
Do that Vdc analysis.
Check out usb scopes or 2nd hand, eg Rapid Electronics 7020 20Mhz 2-channel Oscilloscope | eBay
If you're planning on running vintage amps you'll need to DIY your tech support, if there's none locally.

[Prairie Dawg]

What sort of plate voltage and cathode current are you seeing?

[Mr Pro Fender]

6L6GC Valve
Pin 1 : - 062 into tube from board (by 220K) resistors
Pin 2: Heater 3.0 VAC
Pin 3: B+ in from output 519 (same for both valves)
Pin 4: Screen Grid is 519
Pin 5: -062 then has a disc cap .002 to earth
Pin 6: 519 joined by Screen Grid resistor
Pin 8: 000 to earth

The Volts in to the caps under have all been put in new but when measured they have risen (the two 80 uf) are now 85uf and 86uf. and the three 20ufs two are 30uf and the last one is 26uf. so the volts going into the caps from diods is 519DC (as the GZ valve has been replaced and two diodes put in to convert it so from these two diodes its 519 goes into the big caps under amp (86uf).
Now from these caps the volts out to parts of the board
517 VDC to Choke - reverb tran and trem.
437VDC off to where the B+ enters the driver 'v' two resistors (reading 50K each). (((is 437DC high here)))
and 372VDC off to the two 100K resistors in the pre amp just behind V2.

[Bruce / Mission Amps]

First off... why is there an incorrect rectifier tube in there? It is supposed to have a 5U4GB and use a NOS one... and no diodes.
Fix that first, the plate voltages on yours is way too high now.
Then you can readjust the idle current of the two power tube so they are sitting at around 13-16 watts each. If you don't know how to do that then ask for help.
I bet yours has been modified too... nobody who really know Fender amps and their limitations would put solid state diodes on the tube socket and in the B+ rail, and not also messed up the preamp stages... I'm betting yours is boosted all over with some mod(s) and now sounds like what you are describing.
Regardless, most 60's through late 70's Fender Reverb amps will go into phase inverter preamp tube distortion at +4 on the volume control.
Turn the bass down to around 2 1/2 to 3 max and treble around 5-7. same with midrange if you have it. If it is really distorted, you'll need to do some homework, find and look at a layout of the amp. Compare it to what you actually have....

[Mr Pro Fender]

Thanks Bruce ill have a look - on my Schematic (AA165) it says GZ34 but also read that it's better off with it out for guitar. I was just wondering that off of the dioeds volts seem high (but here in the uk we're on 240Vac). What diodes are ment to be there so I can check if they are correct?

The amp has sounded way better than this not distorting but still had issues with the bass sounding mushy and compressed and the tops rolled off this was 1hour into the set. so the last 20mins of the performance was a poorer sound from this amp. But now it's not a nice distortion at all it sounds like someting is off somewhere or broken?

[Mr Pro Fender]

Pro Pics

[pdf64]

Oh - I can't see any yellow leads for a tube rectifier from the power transformer, and the rectifier socket has been removed and the hole blocked. Maybe the power transformer has been replaced with a type intened for Si rectifier.
Does the power transformer have any markings on it?
In the light of that, the schematic voltages aren't relevant.
Can you confirm the heater voltage(eg across the pilot lamp)?
1st thing is to check that the 6L6 are idling at a reasonable level (aka check the bias), as Bruce advises.

[Mr Pro Fender]

No markings on transformer just the fender date stamp codes at the bottom, output and choke looks same age too same date code
.
I put all 6L6s readings above - Heater was 3.0 VAC on pin 2 and 7.
The bias on pin 1 and 5 are -062 VDC

[pdf64]

What is the date stamp code on the transformers?
Are there any yellow leads on the power transformer (may be taped off)?
Can you re-confirm the heater Vac; 6.0V seems unusually low, Fenders usually run nearer 7V.
We need to know the 6L6 plate dissipation, rather than the exact bias voltage. To do that you need to check the current going through the 6L6. The generally preferred method is for 1 ohm resistors to be fitted in series with the cathode return to ground, to enable this check to be made safely and easily.
Plate voltage x plate current = dissipation.
See
Biasing