HIWATT HI-GAIN 50 Reverb faint and whining on full

[TheAndyEngineer]

I've recently acquired a HIWATT HI-GAIN 50 model tube amp which sounds great

The issue I'm having is that the reverb control doesn't seem to do a lot, There is a very subtle reverb effect but the dial needs to be near full to hear it. With the dial on full a high pitched whining noise can be heard

I've removed and inspected the reverb tank and used a multi-meter to check the DC resistance of each transducer (It appears to be an accutronics model). Both transducers read in the 50-100 ohm range so neither of them appear to be open circuit

I've managed to get hold of a schematic for the amp and I can see 2x 12AX7 and 1x 12AT7 in the signal path of the reverb circuit

How can I further determine what the cause is?

Schematic Here:

http://s000.tinyupload.com/download....09947159009139

[g1]

The schematic you linked has the reverb tank between IC2 and IC3, is that what you have there?

[TheAndyEngineer]

The schematic you linked has the reverb tank between IC2 and IC3, is that what you have there?

Yes thats right, the tank is labelled: 3CA3A1B which when googled appears to be accutronics. I've taken the tank apart and all inside seems fine, transducers show resistance, lead wires can be seen and are not snapped

Ive also replaced each 12AX7 in turn with no difference to the reverb noticed so Im at a loss as to why its not working

[Mick Bailey]

Unplug the reverb and touch a finger on the output connection phono plug with the reverb control at half way. If you get aloud buzz then the recovery circuit is most likely working fine. If no buzz then I check the cable - sometimes they go open.

[TheAndyEngineer]

Unplug the reverb and touch a finger on the output connection phono plug with the reverb control at half way. If you get aloud buzz then the recovery circuit is most likely working fine. If no buzz then I check the cable - sometimes they go open.

Thanks, will try that this evening and report back

Presumably:

Buzz = Reverb tank at fault (or signal path to tank)
No Buzz = Phono cable or reverb driver circuit at fault

[Mick Bailey]

That's right. If the recovery circuit and cable is good, then if you plug in the tray and tap it you should get a strong signal out. If not, either there is a connection issue with the plug/socket or the tray's output coil is defective.

You can also check the drive side to the tray with a pair of cheap headphones (mine are 62 Ohm). Use a couple of clip leads to connect the reverb input connector to one side of the headphones. You should hear the guitar loud and clear, as the reverb driver is a little power amp circuit.

[TheAndyEngineer]

So I've plugged in headphones to the phono jack going to the input of the reverb tank and can hear the guitar loud and clear. Tapping on the phono lead from the output of the tank also makes a buzz the same as tapping a guitar lead so I guess signal path is fine and it's the tank that's dud. To test the tank I played audio from my phone into the input and plugged in headphones to the output, I can't hear any audio from the phone but I can hear the springs when gently tapping them. All solder connections look good so I guess it's the input transducer, can they just go bad like that? What are my options just replace the tank? Any ideas where I can source a replacement as 3CA3A1B isn't getting any results, thanks again! Andy

[g1]

For some reason they are starting the number with 3 instead of 8. However, this 8CA3B1B tank description states it is for that model amplifier.
https://www.tubeampdoctor.com/produc...rb&language=en

[TheAndyEngineer]

For some reason they are starting the number with 3 instead of 8. However, this 8CA3B1B tank description states it is for that model amplifier.
https://www.tubeampdoctor.com/produc...rb&language=en

Thanks for that, closest ive got so far! Only thing bugging me is that tank is input grounded output insulated whereas the tank from my amp is insulated input grounded output

[g1]

Ok, didn't notice that. That's why they have the second listing w/mod for more money.
I know the MOD brand tanks are very easy to change ground scheme, but have no experience with the new accutronics/belton.
I see a ground screw by one of the jacks, I wonder if you can just swap the jacks around.

[Mick Bailey]

Input transducers are an uncommon failure. Usually the output transducer goes open and mainly with the 220 ohm DCR output coils. I would say I get around 1 in 30 trays where the driver coil is the problem. Coil failures can sometimes be found where the winding is soldered to the connector pins. Examine them closely - sometimes there's a break that can be repaired in-situ with a strand of fine wire. Kind of bordering on micro-surgery but it can be done.

[TheAndyEngineer]

Input transducers are an uncommon failure. Usually the output transducer goes open and mainly with the 220 ohm DCR output coils. I would say I get around 1 in 30 trays where the driver coil is the problem. Coil failures can sometimes be found where the winding is soldered to the connector pins. Examine them closely - sometimes there's a break that can be repaired in-situ with a strand of fine wire. Kind of bordering on micro-surgery but it can be done.

I was surprised too but it has to be the reverb tank at fault as I can clearly hear audio when listening to the input phono and I can hear noise when tapping the output phono. Connecting the input phono to the tank and listening to the output of the tank I cant hear anything. I'm sure its the input transducer as when im listening to the output of the tank i can hear the springs if gently flick them

Measuring tip to sleeve on the phono connectors I am seeing:

Output: 69 Ohm
Input: 30 Ohm

In continuity mode the input phono beeps out and output phono does not. I assume this is because 30R is low enough to set off the beeper in my dmm

[Mick Bailey]

I'm not saying it isn't the tank, just that the driver transducers are an uncommon failure. It isn't impossible that the coil reads OK on your DMM but fails under load. They take a fair bit of current to drive and there are magnetic stresses within the coil - neither condition being met when you measure it on the bench. These tanks will work either way round to a degree - you can reverse the input/output connections and see if the reverb improves, though it would be nowhere as much as with the connections the right way round. With the tray connected backwards, just brush the springs with a fingernail to see if there's output generated from the tray's input (if you see what I mean).

[g1]

How loud is the spring noise you can hear from the return side? It needs to be loud. Or try with your thumb on the return cable.
Also, there is a great amount of loss in the tank, so not hearing much with the headphones connected to the tank output jack may be normal.