Two Rock Jet 22 Reverb

[LD50]

New member here, saying hello and asking a question.

I am in the UK and looking at the amp as a favour, I am well versed in plexi era Marshalls, old Oranges, Voxes, Laneys etc but less so with American amps. I have been asked by a friend to take a look at a 1x12 combo as the reverb was not working, he told me that when turning the reverb knob simply created hum.

As typical I did all the simple things first, inspected and cleaned the leads and connections, pots etc and checked for 394v on pin 1 of the driver socket, subbed in another 5751 reverb driver tube, bashed the tank etc. On my testing there is no hum when you turn the reverb pot and mainly the hint of some reverb at max pot setting.
The amp itself works very well except for the reverb issue and is quite a nice (if expensive) bit of kit, well put together with decent component, several years old now and out of warranty, looks like the OT has been replaced at some time and one of the board mounting screws is AWOL.

Hitting the tank you get a crash (not particularly loud) and when you switch the send return leads, suggesting the tank internal transducers are working. Similarly if you remove the 6V6s and take the reverb out from the RCA socket direct to the speaker you can plug into the amp input and play the guitar.

The above suggest to me that the problem is where the signal injects back into the system (power amp). I have cleaned and lubed what I take to be the internal level out trimmer pot which is 1M and appears to be working correctly.

I believe that the amp is similar to a Fender BFDR with pre-amp tweaks but I cannot find an accurate schematic for the Two Rock which I would like to have before I lift the board and dig my scope out.

Links to

[Stevem]

No hum when turning up the reverb, or no loud crash when banging the tank means that if nothing else the output transducer coil is open on the tank, and or the recovery amp side of the reverb circuit is not working.

Here are couple of things you can do to prove this out.

1) you can make up a jumper wire with a 4700 ohm resistor in its center lead to jump across the reverbs RCA jacks.
If all is well circuit wise then turning up the reverb will act as a volume boost circuit, as in the send amp is sending and the recovery is reciving.

If both the input and outout side of the pan are at ground then you only need to jump the center connection of the amps rca jacks thru that 4700 ohm resistor.
2) hook up your voltmeter set for ac volts to the send jack and look for a voltage output to show up when you play or pump signal thru the amp.
To test the recovery amp pull signal off of a preamp stage plate resistor and feed it thru a .0 whatever cap rated for 400 volts or better into the return rca jack, and once again listen for volume gain when you turn up the reverb.

All of the above assumes that either end coil in the pan is not open.

I have had many pans that looked perfect yet one of the wire connectiin on the pans rca jack was broken off yet since it was hidden looked good!

[LD50]

Thanks for the quick response. Sounds sensible (so typical pan is 4k7 passive resistor).

I will have to complete to days 'Honey Do' tasks but will take another look later.

[martin manning]

...so typical pan is 4k7 passive resistor.

No, but putting a resistor there would avoid blasting the recovery stage with the full output voltage from the driver transformer, since that resistor will form a divider with the grid leak on the recover stage. The grid leak is probably quite large, so with 4k7 you will still be hitting the recover grid with a lot of voltage. I would measure resistance across the input and output transducers to see if one or the other is open, and also check the resistance from the input shield to the output shield on the tank to see if the common connection has continuity. Some tanks do not have a through ground, so also check the shield resistance to ground on the amp's send/return in case the common is in the amp.

[LD50]

Thanks

I subbed in the resistor and made sure there was a proper ground to the shield

The pot acts a a volume control now, so I guess that a new tank is in order. It is a fairly short Belton device, IIRC 5 springs?

[sluckey]

I would try some different cables before buying a tank.

[LD50]

A belated thank you.
It was the cables after all (intermittent)
New ones installed and amp is functioning normally.
I learned something in the process so all good
Made me contemplate making my own BFD but with normal channel and gain channel (instead of vibrato) but too many other projects to finish first!