Balder lightning guitar amp schematic please!

[nickb]

R26 went from 39k to 33k and the volatge went from 0.6mV to 0? That's crazy. Measure the voltages around Q6 and check the value of R26.

[MarkusBass]

I think it's (almost) correct. The fact that with 100 Ohms as R26 you get 25mV doesn't prove that with 1.3 Ohm you should be able to get 25mV as well. We discussed this quite a long time. Could it be that the drop from 0.6mV to 0.0mV is caused by meter accuracy?
I would solder another 33k resistor on top of R26 (to get 16k as R26). The simulation showed that the value of R26 should be as low as 12k. Let's try 16k for now. Unless, 33k is not soldered correctly now .

Mark

[nickb]

I think it's (almost) correct. The fact that with 100 Ohms as R26 you get 25mV doesn't prove that with 1.3 Ohm you should be able to get 25mV as well. We discussed this quite a long time. Could it be that the drop from 0.6mV to 0.0mV is caused by meter accuracy?
I would solder another 33k resistor on top of R26 (to get 16k as R26). The simulation showed that the value of R26 should be as low as 12k. Let's try 16k for now. Unless, 33k is not soldered correctly now .
Mark

Note of caution: If you do this please raise the variac very slowy in case the iddle curent is too high. I couldn't bear to see a grown man cry

[MarkusBass]

Another note of caution: you wanted to have DC conditions of the amp correct. But now I see that Bigdrums provides input signal and get full power out of the amp. Shouldn't he wait for all DC voltages be correct (including voltage on R26)? Unless, we agree that the amp works in class B and 0mV on R26 is OK.

Mark

[dmeek]

I suggest a slight change to the bias circuit. This way if the trimmer fails the idle current goes to minimum.


http://music-electronics-forum.com/a...1&d=1507643168

[bigdrums925]

OK I put another 33K on top of R26 and touched up 1 solder joint that looked bad
Now the range of the trimmer is 3.3mV to 5.6 mV
The trimmer seems solid per watching the VOM

dmmek: your suggestion is noted and I think I should wait for nick and markus to tell me when. Thank you

I just plugged in a guitar and speaker and the amp sounds as good as I might hope for. Both gain and master are at 1/2.

Awaiting instructions

[MarkusBass]

This is the question to Nick. How much current would you like to have? For the start I wouldn't go higher that 10-15mA. What do you think?
Do you remember that my simulation showed that value 12k seems to be a good value for R26? Now we have 16.5k. Should we change it to 15k, or 12k? (But remember that with 12k you shouldn't check the maximum current because it will be 30mA, or more).
Dmeek's modification is excellent but maybe we could wait and check what we get with the 15k resistor?
Bigdrums, I assume that now we have all power transistors soldered, all 1.3 Ohm resistors soldered and you are trying to set the bias of the amp (its quiescent current), right? The power transistors are attached firmly to the heatsink and variac is fully on. yes?

Mark

[nickb]

Doug's idea is a good one. Go for it!

We could probably go lower for R26 yet. Before we do that feed a 1Khz sine wave in and set the controls for say 10Vpp with a load connected ( got a dummy load?) and look for any signs of cross over distortion. If it looks clean and it sounds good to you I'd be tempted to leave things as they are. Otherwise, try a small value for R26, say 15k and try again. Remember to always set the trimmer to the R25 end to start with.

Mark: The magic number is around 26mV as at this voltage at which the output impedance of the transistor is equal to the 0.65 ohm emitter resistor. But it is just a guide and without any other information I'd choose it as a do not exceed value. Anything less than that where (a) there is no crossover distortion and (2) the idle is current stable when the amp is hot works for me.

[MarkusBass]

Currently, Bigdrums has 33k resistor soldered on top of another 33k (it gives 16.5k). I'm sure this does not look nice and I would replace R26 with a single 15k resistor.
Thanks for explanations regarding the current.

[bigdrums925]

I put a 15K in R26 and dissipation is now 7.1 mV max until I put a signal through it.

Hard to check for crossover distortion as my scope gets fuzzy as I turn up the volume but from what I can see it's fine

It works well with the guitar and speaker hooked up

I think we should call it good.

Should I add Doug's jumper?

[J M Fahey]

I think if you can play normally, plus you can play LOUD for an hour and amp does not self destroy, then better stop repairing it and put the cover back

[bigdrums925]

Thanks to all that contributed to the successful outcome of this restoration, I can't thank you all enough.
There is a bit of buzz affected by the master volume only and I need to get the reverb working again
But without this website and the tireless efforts by all, this amp would have probably never worked again.
You guys are awesome!

I will graciously attempt to make the last few changes to the schematic for anyone that may need it.

[MarkusBass]

There were still some doubts regarding the feedback resistor (220 Ohms) but if the amp works OK, I would leave it as it is. Happy playing.
What about a final photo of the assembled amp? Just for goodbye .

Mark

[nickb]

Here is the final power amp schematic.

[bigdrums925]

Mark: Absolutely, be happy to post pictures, give me a day or 2 as I put the unit back in the combo box but will have it out soon.

Nick: Thanks so much for the updated schematic. I think I unintentionally made things difficult for you guys, but we got through it! I am so happy about it.

How can I ever repay?