1969 Pro Reverb -

[olddawg]

I would really suspect the power supply filter caps if they are not fresh and then would try a different set of speakers. Do you have any way to check the output symetry and measure the clipping level at various frequencies? Ie. A scope and a dummy load?

[Mr Pro Fender]

Caps are all new though they seem to haved drifted up a little?
Trid different speakers - still the same
scope and a dummy load - Im looking at building this set up but the cost of a scope at the moment - I have a hand held one but unsure if it will do the job.

[Mr Pro Fender]

The wiring for sure is old and the wires seem hard not flexible - I have changed all the caps under the amp but when I tested them they had drifted up. the 2x 80uf were around 85uf and 86uf - the 3 x 20ufs are 30uf 30uf and 26uf.
Unsure if these are ok as drifted.

Where is the signal Coupling cap and you mean I need a scope (I have the handheld one if that would do the job?)

[pdf64]

Electrolytic caps generally have a wide tolerance, say +/-20% so their value is in spec; however caps may have faults that prevent them working correctly, yet could still have the value in spec. So as you can't currently measure that they're doing their job (the Vac on the VB+), subbing in know good ones may be your best way of confirming them.
Signal caps - you may be able to check whether they're open circuit just with your meter (or scope with a 10x probe) and something to generate a constant tone. As you don't have a dummy load the noise would drive you/family/neighbours nuts, but for this purpose you could remove the 6L6 to quiet it.
Feed in a constant tone (eg from keyboard etc, lower than middle C) and adjust the keyboard volume to get about 0.5Vac at the first grid V2pin2. Check plate voltage pin 1, should be 20-25Vac.
Turn bright off / treble & bass down mid up, adjust amp channel volume to get 0.5Vac at V2 grid pin 7, check plate pin 6, should be about 25Vac. If lower than that, remove V3 as that can clamp the positive going wave to 10V peak (ie V3 cathode voltage). Then follow the wire from V2 pin6 to the circuit board. Check the voltage on the near side of the brown cap, then on the far side, should still read 25Vac.
Then adjust amp channel volume to get 0.5Vac at V4 grid pin 7, check plate pin 6, should be about 20Vac. If much lower than that, remove V6 as that can clamp the positive going wave.
Then follow the wire from V4 pin6 to the circuit board. Check the voltage on the near side of the brown cap, then on the far side, should still read 20Vac.
Refit V6, if just removed.
Then adjust amp channel volume to get 0.5Vac at V6 grid pin 2, check plate pin 1, should be about 10Vac, also pin 6 should read near enough the same.
Then follow the wire from V6 pins 1&6 to the circuit board. Check the voltage on the near side of the brown caps, then on the far side, both should still read 10Vac.
Hope that makes sense -good luck!

[52 Bill]

I've got a question for you, how are you setting the bias on the output tubes? Are you reading the idle current in any way, or are you just setting a voltage at the control grids (pin 5)?

[Mr Pro Fender]

Ill do some checks later today and post back around 9pm GMT tonight. Hope that's ok bill -
Ill do them other tests above saw my hand held scope in action todays seems ok I have a 10x on it. depending on noise tonight it might have to be tomorrow. Though today it seems that the scope when he put it on the Bias caps the two 220 'V' reisitors the sign wave was poor when turned to 4 on the volume. he was telling me he thinks the problem is in the driver Circuit or feedback Circuit (we tried two new JJ 6L6s) still there changed feedback resistor and the two plate load resistors '^' (below the bias 2x220 'V') left changed to 100K and right side 82K. still there. rang another friend who's old school he said to look at the fender 75 and see what pin 3 and 5 are doing, as this amp seems to have 519 volts on pin 3 which seems high????. (havent done this yet). the guy today spent 4 hours with the amp and me and still couldn't fixed it - I felt it may of been a bit beyond him as he dosnt do many amps.
Thanks everyone and Pdf64 with sticking with me on this - i'm sure we'll get there. back later.

[andyjazz]

I have a 64 Pro Reverb. About 7 years ago it started sounding all distorted and dark and blowing fuses. (I kind of liked the darkness BTW) Anyway it had the original filter caps that needed to be changed. I also experimented with changing some of the tone caps. I've ended up with the original tone caps, new cathode bias caps and filter caps, new tubes and biasing and new JBL's. The amp is REALLY clean and LOUD. It is almost too much for the small clubs I play in. I have to play it with the treble knob on "2." I am surprised that you would play a 69 Pro at volume 8. I've never run mine over 5, and that was when I was in a classic rock band playing Pink Floyd, ZZ Top and Zep in bigger venues.

Anyway I didn't see in the posts if you've changed the filter caps, it's the first thing I check on old Fenders. Also I don't know what you've arrived at as far as the rectifier issue goes. I really like tube rectification on Fender amps. I have a polytone for when I go for that whole SS sound. Anyway, Good Luck!

[Mr Pro Fender]

Hey Andy

Thanks man, I had changed filter caps. Also the three tone caps. new cathode bias caps well I dont think these have been changed - I have tested all caps and they are all in spec. Tried different speakers and valves (from my other working amp). So right now just waiting on some test things to come in post so ill be posting some new test stuff back up soon. Does anyone know how to build your own dummy load for 100Watt that should cover my twin and this 40 watt pro?

[MarkusBass]

Does anyone know how to build your own dummy load for 100Watt that should cover my twin and this 40 watt pro?

You can few buy resistors like this one: RH05016R00FE02 Vishay/Dale Wirewound Resistors - Chassis Mount and connect them in parallel.

Mark

[Jazz P Bass]

Bear in mind the resistors should be mounted to a heat sink.

[Mr Pro Fender]

that's great - though i've never built one before - is it that simple just buy say 4 of these resistors and wire them in parallel.i know they get hot and need to be on a safe base. I guess then a speaker jack is needed which is connected to the resistors with speaker cable connect the two ends from the resictors to each cable. what about the ohms of the speakers your subing from output transformer - does the dummy load need a switch to go between 4ohm - 8 ohm and 16 ohm?????

[Bruce / Mission Amps]

I've never seen that PT but might be original... it is an amp for use outside the USA.
It is possible that the PT is wired to use a GZ34 but, in my opinion, the voltages are too high with the SS diodes anyhow.... I can't see if the PT has a 5V tap or not.... a lot of that work looks unoriginal.
What is your actual line voltage there? I UK buddy of mine told me his is close to 255vac some days... that would make a huge difference if the amp is supposed to be using a tube recitifier at 120v to 220-240vac ... compared to a solid state diode rectifier setup at +250vac.

[Mr Pro Fender]

Here is a drawing of the readings from the Transformer. I have no way of knowning what Dioes have been used here?

[pdf64]

Bruce was right then, your line/mains supply is a little high.
Does the voltage selector have a 260V position? Some do, others stop at 240.
If you're keeping with silicon diodes, it's probably best just to replace them, then we know the spec of what's in there.

[Mr Pro Fender]

No it stops at 240. whats best thing to do here? what diodes do I need if I need to replace them? not bought them before. this cant be the problem as the amp as worked and sounded ok i'm guessing these Vac would have been around the same voltages all the time. but it would be good to get it close to +440Vdc as on AA165 schematic layout as to my 519Vdc I have right now.

Ive changed the old cathode caps (25/25) also the old phase input cap (.001) ive replaced - still distorted past 4?