Hiwatt CP103 build

Share your home builds, knock offs and ground up customs.
shakti
Senior Member
Posts: 2042
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: Hiwatt CP103 build

Post by shakti » Fri Jan 11, 2019 11:02 am

I think I may be on the right track now...

It looks as though the preamp is good to go after my previous troubleshooting. The output from the PI looks balanced and decent. The amp is also loud enough, but sounds like crap, so the signal driving the output tube grids seems to be sufficient.

Moving on to the power amp then. As there seemed to be some kind of imbalance in the output, I decided to simplify Things and try to run the amp with just two power tubes, one per side. I set the impedance selector at 8 ohm to adjust reflected primary impedance accordingly. I adjusted the bias so the dissipation reading via the 1 ohm bias reading resistors came to about 40 mA each (plate voltage 480ish with just two power tubes). Put the two tubes in V6 and V9 respectively. Turn amp one with the chassis upside down, so I couldn't see the power tubes well. Amp still sounds like crap. But after just 20-30 seconds of playing, a loud pop and a flash on one of the power tube sockets (V9, on the side that seemed to dissipate a lot). Sound disappears.

Turn amp off. Check for burn marks on sockets and tube, none to be seen. Check fuses, none of them blew. Then I go checking on the grid stop, screen grid and bias reading resistors again. Turns out the 1 ohm bias reading resistor was the one that popped and blew. But strangely enough, one of the resistors on the OTHER side (V7) is now also bad, but not the one where I had the tube on the other side. I had checked them before and they seemed fine.

I check the DC resistance between CT and each primary wire again. Same result as before, about 15-16 ohm on each side.

The power tube on the "hot side" must have been dissipating a lot since the 1 ohm resistor blew. Perhaps we are on to something now - if there's a gross imbalance between power tube dissipation, I would get a lot of crossover distortion, right? And if the bias reading resistors are badly imbalanced or on the brink of going out, that could probably account for it. These represent the ground connection for each tube, and if contact is intermittent or badly imbalanced, that could probably account for some of my problems.

So I yank the resistors out, and replace with a regular ground wire. Check that I have good continuity to ground on pin 8 and 1 on all power tubes. Then I try to test it again with four tubes this time. I don't have a tube tester so I don't know if the power tube in the socket that blew a resistor survived, but I try to put all four tubes from the quad back in. Then I just turned the bias adjustment pots down to maximum negative bias voltage on both sides just for precaution.

Power up and turn amp on, but before I get to play anything, I see a power tube redplating, this time in V8. Turn the amp to standby, and check bias voltage. -42 on V6 and V7, -7 on V8 and V9.

WTF?? This is where I ran out of time before I had to go to work, but now I think we are getting closer to the root of the problem. Maybe some kind of intermittent problem in the bias circuitry? Remember, I have dual bias pots where I set the range for each bias pot with some balancing resistors, so there's actually a lot of potential for things to go wrong. However, at idle I have, up until now, never seen any problems either with reading bias voltage or power dissipation. But perhaps when it begins playing/conducting/vibrating that's when problems show up.

I need to really go over the bias circuitry now. But this has got me thinking about those 1 ohm resistors I have put in all my amps. Are they such a good idea after all? If they go bad, what could happen with the sound? If they are imbalanced, what does that do with the sound. On the other hand, as it looks like in this case, they may also represent a sort of safety valve/fuse function, and burn up in the case of over-dissipation.

Any thoughts from you guys who know much more than I do?
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

User avatar
neikeel
Senior Member
Posts: 7157
Joined: Tue Dec 06, 2005 8:31 am
Location: Suffolk, England

Re: Hiwatt CP103 build

Post by neikeel » Fri Jan 11, 2019 3:11 pm

As long as the ground connection is good and the 1ohm resistors test 1 ohm (+/- 1%) then I see no problem they are probably acting as a fuse too!

Did you try swapping the pair of output tubes and the redplating does not follow the tubes?

What OT did you use?

Have you lubed the bias pots and checked the bias caps on that side?
Neil

shakti
Senior Member
Posts: 2042
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: Hiwatt CP103 build

Post by shakti » Fri Jan 11, 2019 3:43 pm

Haven't got to that. The redplating tube was not the same that was in the socket with the popping resistor, but it was on the same side of the OT.

Through all of this, V6 and V7 seem to be conducting less than V8 and V9. The blown resistor was on V9 and the redplating (that I have seen) was on V8.

V8 and V9 were the sockets where I needed to put more negative bias voltage to keep the bias in range than on V6 and V7 (at idle). All of these problems have followed the sockets (V8 and V9) and not the tubes.

There is only one bias cap, but a network of resistors and a pot for adjusting the bias voltage to each side. I will go over every detail in that network.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

shakti
Senior Member
Posts: 2042
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: Hiwatt CP103 build

Post by shakti » Sat Jan 12, 2019 4:42 pm

Problem solved! So what was the problem...?

Let's see...last we checked, I had a problem With the bias voltage on V8 and V9. This was in standby, where bias voltage was a measly -7 on those two sockets. HOWEVER, I also had the tubes in, and that included the tube that was in the socket where the 1 ohm resistor blew. Once I removed the tubes, bias voltages were fine again and the same across all sockets. So the tube must have taken a fatal blow.

OK, so now we have a problem with uneven dissipation, problems with redplating and blowing tubes on one side. I check all the bias circuitry, and it looks fine, except for one small detail: Where it divides into the two sides, I had two solid core wires very tightly and neatly wrapped around the turret, but they had not been soldered!! I had been running it like this for 8 years not knowing and without any apparent trouble. I still don't know if that was the culprit, because bias voltage and dissipation has been apparently steady up until my recent trouble. But it is certainly thinkable that over the years, oxidation had developed and/or mechanical vibration caused the contact to become intermittent or poor on one of the wires (most likely the one that serves V8 and V9) causing fluctuations in bias voltage, insufficient bias and resulting redplating and over-dissipation.

OR...one or more of the 1 ohm bias resistors went bad/out of spec/whatever causing inaccurate dissipation readings, bad biasing, crossover distortion...whatever...and then finally one of them blew.

I suppose bias voltage problems is more likely at least as a start, but it does look like a faulty 1 ohm resistor was at least part of the story at some point into this.

After soldering the two bias wires, removing the 1 ohm resistors and getting a new set of tubes in (sadly the redplating Mullard that started all this looks to be past its sell-by-date now...), the amp is back to sounding normal. It sounds great, but still not a good match with a Strat. I am going to A/B with the output stage from my DR103, that did seem to sound overall better but a little darker. The spank and brilliance from this amp is something to behold.

Oh, and for whatever reason, the oscillation on channels 3 and 4 is now gone after resoldering the input grids for the umpteenth time. I didn't do anything different this time, so I really don't know why it worked this time...*unless* it also had something to do with the instability in the output stage. We will probably never know.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

User avatar
neikeel
Senior Member
Posts: 7157
Joined: Tue Dec 06, 2005 8:31 am
Location: Suffolk, England

Re: Hiwatt CP103 build

Post by neikeel » Sun Jan 13, 2019 8:03 am

Glad you got it sorted.

Also pleased (in some respects) that there wasn't anything missed in the trouble shooting.
Neil

shakti
Senior Member
Posts: 2042
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: Hiwatt CP103 build

Post by shakti » Sun Jan 13, 2019 4:43 pm

Many thanks for your moral support and suggestions! I am still a little confused about how elusive this turned out to be, given that the first things I checked were bias voltage, dissipation and all the resistors on the sockets...and the problem turned out to be either or all of these! :lol:
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

Post Reply