KOCH amp help

[Valvehead]

I am trying to repair a Koch Twintone 1 .

the 1st problem is the amp has very low volume. I have checked a lot of things :


swapped all tubes
bias ok at 33mv
all voltages look normal at all tubes
B+ is 445V
power soak resistor ok (even tried unplugging that soak)
re-seated all 3 op amp chips
cleaned all jacks
cleaned all pots

I didnt find any open resistors
the 2 screen resistors are ok

the amp still has low volume - sounds like about 2 watts instead of 50 w !

One thing i tried-- i ran the koch effect SEND to another amp and it had FULL loud volume . So that means the problem is somewhere AFTER the FX return .

Could it be one of the IC chips ? or transistors (T1 , T2) ?



Problem #2 might be related , the amp has a loud hum. Even with all volumes on 0 . The hum goes away when the phase inv. tube is removed ---this might be related to the low volume problem


trying to attach the scem here

[Valvehead]

EDIT- i did find something strange tonight . Measured all the heaters and they all show 6.x VAC .

BUT , on V1 only :

I cant get any AC readings on pin 4/5/9 .

pin 4 - the ac meter just bounces around , also shows a couple millivolts dc

pin 5 shows 13.7 VDC

pin 9 shows 6.8 VDC

tried swapping in a new 12ax7

[Robert M. Martinelli]

That's normal, if you look at the schematic, you can see that the heaters of V1 are series-supplied (pins 4-5) with DC (to eliminate any heater hum issue on this very sensitive and important stage), so swapping the tube does nothing. Pin 9 is the center pin for both heaters (used for 6.3V "parallel" supply) so it's normal to read half of the heaters' supply voltage on it.

Hope this helps

Best regards

Bob

[olddawg]

Are you absolutely sure that those output shorting jacks on the speaker out aren't dicked up? I would float them and and solder a speaker load directly to the OT secondary. Then.... put a scope or an AC voltmeter across the terminals with a signal going into the effects in jack and roughly calculate the power. There are many on this board with much more tube amp experience than me, but I think that would be a good start. You might have to suspect the OT.

[Valvehead]

That's normal, if you look at the schematic, you can see that the heaters of V1 are series-supplied (pins 4-5) with DC (to eliminate any heater hum issue on this very sensitive and important stage), so swapping the tube does nothing. Pin 9 is the center pin for both heaters (used for 6.3V "parallel" supply) so it's normal to read half of the heaters' supply voltage on it.

Hope this helps

Best regards

Bob

yep---i see that now , on the scem i posted. The scem i have on my bench (1995) doesnt show the heaters

[Valvehead]

Are you absolutely sure that those output shorting jacks on the speaker out aren't dicked up? I would float them and and solder a speaker load directly to the OT secondary. Then.... put a scope or an AC voltmeter across the terminals with a signal going into the effects in jack and roughly calculate the power. There are many on this board with much more tube amp experience than me, but I think that would be a good start. You might have to suspect the OT.

ill try to bypass the jacks , i did clean them and test them as far as continuity .

[Valvehead]

Tried OT directly to a speaker cab ---same thing , low volume.....

[Alex R]

- if you can feed in a steady signal and turn the thing up you could take a look what kind of max AC voltage you can put onto the power tube grids. Should be able to get a ball park of 20vAC or more.

[Valvehead]

- if you can feed in a steady signal and turn the thing up you could take a look what kind of max AC voltage you can put onto the power tube grids. Should be able to get a ball park of 20vAC or more.

with a 200hz / 1k tone gen , i get 7vac across pins 4 , and 11vac across pins 5 - thats into the Input jack.


when i go into the fx return , i get 9v and 15v

[Alex R]

Sorry didn't mean pin4 screen grids. Those pin5 figures are kind of lowish - I am assuming they are the maximum you can get. Not very decisive figures but we might look at the preamp now. Again with the signal at a maximum you should be looking at more than 1vAC going into and out of the effects loop. Check at R42, where we would like at least 1vAc or more, and R60, where the PI should be being fed a somewhat higher figure after the effects return buffer opamp.

TR1 and TR2 are reverb drivers, not the problem.

[Alex R]

...maybe someone cleverer than me can explain what sort of PI that is and how it works?

[Valvehead]

Sorry didn't mean pin4 screen grids. Those pin5 figures are kind of lowish - I am assuming they are the maximum you can get. Not very decisive figures but we might look at the preamp now. Again with the signal at a maximum you should be looking at more than 1vAC going into and out of the effects loop. Check at R42, where we would like at least 1vAc or more, and R60, where the PI should be being fed a somewhat higher figure after the effects return buffer opamp.

TR1 and TR2 are reverb drivers, not the problem.

getting 12vac and 10vac on each end of R60

getting 11vac at r42

with a 1k test tone in the input jack --all volumes / gain on 10


i posted this before , but i ran the fx send into a good amp and it was very loud and proper . i did the same on the koch fx return and had very low volume

[Alex R]

- starting to sound like the OT. Try patching in a spare if you have one. If not, check plate-to-CT resistances are within around 30% of one another.

[Valvehead]

plate to ct is 28 ohms and 31 ohms

plate to plate is 51 ohms

i will have to tear apart another amp and try and clip in a OT , but all i have are 100w amps here

On a side note, the Koch guy said these amps did have jack issues, so they sent all new jacks, they are all installed , but made no difference .

[Valvehead]

Ok,, i clipped in a good OT from an Orange amp.

still low volume and hums. You can talk over the amp when its on 10 / both channels