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EEApprentice 10th November 2012 04:00 PM

PP KT88, Need help!
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I've been lurking this forum for quite a while now and have made 2 amps from designs I picked up here. They all worked perfectly so I decided to try to make my own. I'm an electrical engineering student so I thought I could tackle the task. I've spent the past few months designing and constructing this amp but last week I turned it on to find that I completely failed.

This is a stereo, monoblock PP KT88 Class A amp. I've spent the last week trying to figure out what's wrong with it but I cannot, for the life of me, find anything that fixes the problems. Please, if someone here could take a bit of time and maybe there's something that I'm overlooking by lack of experience that you could let me know?

I'm attaching the full schematics as well as pictures of my actual circuitry.

The list of problems:

- Right channel is reproducing music veeery quietly.
- Left channel had an awful loud distortion/hum (no music playing and even when the potentiometer was off). This went away just by moving wires near the power supply, but still no music playing.
- Measurements of my B+ are somewhat close on my right channel, but fluctuate heavily on the left one. When I could hear the noise I was getting 410 [V] right after the stand-by switch. After moving wires and no hum, it's at 565 [V].
- If I move the potentiometer my power supply voltage increases

Thank you very much in advance to anyone who could take to try to help me out. :(

payitforwardeddie 10th November 2012 04:34 PM

Just a couple of suggestions. The wiring is a bit of a rat's nest. Have a look at some other amps that have been worked on by others here for examples of neat wiring. Also, using different colours will help indentify errors. I would fit "Grid Stopper" resistors (eg 4k7) to each control grid close to the valve base to stop any odd oscillations. Also, temporarily remove R7 to remove the feedback ( in case its positive instead of negative). One more thing, try removing the coupling caps to see if each stage is stable on its own.

DeathRex 10th November 2012 04:57 PM

Use solder more, screws less. Try to get away from flying connections. Get some terminal strips 5-Position Terminal Strip : Terminal Strips |

Keep components close to each other; like where the preamp is, the only wires going to it would be the audio in, ground (from your one signal star ground), B+, feedback, and audio out. All components should be directly connected. The power amp tubes should be close to the output transformers, with just an audio in wire, ground, anode to transformer, screen to transformer.

Is one hella chassis.

Would be much easier with the component values on the schematic.
It would be 30 volts on the cathode, instead of -30? With 30 volts across a 150 ohm resistor that would be 200ma through the KT88, or about 80 watts?

DeathRex 10th November 2012 05:16 PM

Here's one with too perfect wiring. You don't want the signal wires compressed together, with each other or B+, except if they are all the same level (like all audio inuts). Other than that, his builds are excellent. And you'd rather have filament wires, even when twisted to run perpendicular to signal wires. Morgan Jones' Valve Amplifiers 4th edition would make for some excellent reading.

Well Come My WebSite Audio Collections

DF96 10th November 2012 06:05 PM


Originally Posted by payitforwardeddie
Also, using different colours will help indentify errors.

Yes. Did you get a good deal on red wire?

EEApprentice 11th November 2012 06:05 PM

DeathRex: Yeah, I've been using Morgan Jones' Valve Amplifiers almost as a Bible haha.

DF96: Yup! I bought a 100ft spool, but I don't know if its too thick... Soldering 16 awg wasn't easy.

I'll try to re-do all the wiring... I think this is kinda good news, I was worried someone might look at the schematic and say that I made a mistake on my circuit design (meaning a more expensive fix)...

Thanks for the responses! If ya'll got any more advice/tips/comments I'd appreciate it!

dgta 11th November 2012 10:36 PM

Re-post your schematic with all component values and all DC voltages as measured. Connect a 1Vrms 1khz sine wave at the input and measure and post all AC (signal) voltages on the schematic.

Faults will likely be obvious.

EEApprentice 12th November 2012 02:17 AM

dgta: I'll do that!

I noticed my heaters' center tab wasn't grounded, so I did, but the problem isn't gone. Two things are happening right now:

-One channel has a crazy loud hum (even if the pot is at off). If I turn the pot, the hum changes.
-I'm getting the right B+ now but when I turn the pot for more volume (even without music playing), that voltage goes higher. Is this normal?

dgta 12th November 2012 02:27 AM

See my previous post.

As an EE student, you should know by now that you need to go about this in an orderly, organized manner. Don't jump around trying this and that at random. Start by measuring all DC voltages at quiescent (shorted input). Post that and fix all obvious discrepancies.

Once that's done, connect a signal and make sure there is no DC on that signal source. Then measure AC signals down the line. Again, faults will be obvious.

Vulcanbb18 12th November 2012 02:43 AM


I'm still pretty new at this too, so no doubt someone will correct me, but in your schematic your phase splitter (2nd half of V1) has no grid leak resistor, so the grid is just floating?? (has no ground reference)... I would imagine this would lead to some pretty funky effects... maybe try a 1M or 500K resistor from the grid to the top of R6?



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