Very Low volume problem

[zx11]

Hello I got a metro amp superlead drop in board to rebuild a 90s superlead reissue. I also switched it to a superbass and switched it to kt88s. It sort of works but the volume is about as loud as a tv set half way turned up. Here is a link to some high res photos. I know the wiring is not the best, this is the first tube amp I have worked on and I should have picked a simpler project.

any ideas?

thank you

Index of /amp

[Enzo]

Can you post or link a schematic for just what you built, so we can refer to specific parts?

Have you tried different tubes in the amp? Are BOTH heaters lit in all the small tubes?

Whenever someone has such trouble with a home built or major modified amp, the first concerns are missing connections and wrong connections.

Basic checks would be to pull all the tubes, and check for B+ voltage on pins 3 and 4 of the power tubes, and pins 1 and 6 of the preamp tubes. And of course bias voltage on pins 5 of the power tubes.

With tubes in, I'd expect to see a lower B+ voltage at pins 1 and 6 of each preamp tube, except any cathode follower stage. And I'd expect to see some voltage on each cathode. A lack of cathode voltage generally means that triode is not conducting.

[zx11]

thank you, sorry for the double post, when the first one didn't show up I posted a second time
I tried different preamp tubes, I only have one set of kt88s though. the filaments are all glowing.
I measured the voltages of the tubes here they are, the v7 pin 8 seems wrong. but I cant figure out why. thank you

Tubes Out

V1 Pin 1 443
Pin 6 443

V2 Pin 1 477
Pin 6 452

V3 Pin 1 455
Pin 6 455

V4 Pin 3 475
Pin 4 473
Pin 5 -52

V5 Pin 3 475
Pin 4 475
Pin 5 -52

V6 Pin 3 477
Pin 4 475
Pin 5 -52

V7 Pin 3 477
Pin 4 476
Pin 5 -52


Tubes In

V1 Pin 1 134
Pin 2 0
Pin 3 1.95
Pin 4 2.94
Pin 5 2.94
Pin 6 136
Pin 7 0
Pin 8 1.95
Pin 9 2.89

V2 Pin 1 160
Pin 2 0
Pin 3 .92
Pin 4 2.92
Pin 5 2.92
Pin 6 275
Pin 7 160
Pin 8 160
Pin 9 2.87

V2 Pin 1 323
Pin 2 62.3
Pin 3 73
Pin 4 2.93
Pin 5 2.93
Pin 6 204
Pin 7 23.5
Pin 8 73
Pin 9 2.89

V4 Pin 1
Pin 2 2.94
Pin 3 442
Pin 4 437
Pin 5 -52.9
Pin 6 440
Pin 7 2.88
Pin 8 .07

V5 Pin 1 .07
Pin 2 2.95
Pin 3 442
Pin 4 437
Pin 5 -52.9
Pin 6 440
Pin 7 2.9
Pin 8 .07


V6 Pin 1 .07
Pin 2 2.98
Pin 3 443
Pin 4 437
Pin 5 -52.8
Pin 6 440
Pin 7 2.95
Pin 8 .07

V7 Pin 1 27
Pin 2 3.02
Pin 3 443
Pin 4 442
Pin 5 -52.8
Pin 6 440
Pin 7 3.0
Pin 8 27

[acorkos]

swap V6 and V7 and see if those 27V readings on pins 1 and 8 are on the socket of V7 or the tube you moved from V7 to V6

[zx11]

ok I swapped them. the 27v is still on the v7 socket.

thanks

[Enzo]

I am guessing V7 is one of your KT88s?

Pin 8 is the cathode, and should be at essentially ground just like V6. It looks like you either have a missing connection to ground from there, or if you used 1 ohm reswistors as bias samplers, you either burnt one out there or it was a wrong part in your 1 ohm bin or some such.

Or you miswired pin 8 on that socket perhaps.

[zx11]

pin 1 and 8 are connected and then go through the resistor to ground. I will disconnect the resistor and test it tomorrow morning. thanks

[g1]

As has been mentioned, V7 does not seem to be running correctly, either due to a bad connection or bad resistor between pin 1&8 and ground.
However, removing 1 out of 4 output tubes is not going to cause the kind of major volume drop you have described. There is probably another problem.
Please post the schematic.

[Enzo]

That's right of course, it is just something that needs to be corrected.

[zx11]

The resistor was bad, it was measuring 2.5m, I changed it. The amp is louder now but not as loud as a 100 watt amp, more like a 20 watt. It would keep up with a drummer but it doesn't shake the building if I turn it all the way up.

this is pretty much the schematic http://metroamp.com/wiki/images/c/cc/1992_July_70.gif
but there are some differences like the kt88s, and the bias adjustment resistor.


thank you

[zx11]

I was just reading the ruby tubes site and these tubes have a maximum dissipation of 42 Watts. could the low volume be
because the bias is set incorrectly. its at 53mv now but it should be 66mv to be running at 70%. or do I have it wrong.
thanks

[Enzo]

No.

Imagine you have a car with 100 horsepower, and for some reason you can't get it to go more than 5 miles per hour. You find that in reality it is only making 86 horsepower. That may be something needing correction, but is hardly the reason for 5mph.

If your amp is really quiet, it is not a matter of power being a few watts off. If you cut power in half on an amp, it only drops loudness 3 decibels.


DO not confuse idle dissipation in the tube with audio power the tube can generate. They are separate things.

[g1]

You have a very large difference between the plates on your phase splitter tube (V3). 323V on one plate and 204V on the other does not seem right. I would think the side showing the 323V may not be conducting.

[zx11]

I think I am going to take out the board and rewire it, and do a better job. I have been being lazy and putting it off, thinking I would just get it working, but I think it needs to be done. thanks I will post back if it works

[zx11]

I took out the board and rewired it, and it works now. I don't know what was wrong but whatever it was it seems to be resolved.

thanks