ultratone SE 6bq5 stereo amp - mystery guy

[Jcricket]

Okay folks,
I went to town on this little amp. I do have a question.

There is a 100uf 15v non-polarized cap under the hood. It connects from both of pin 3's on the 6bq5's to ground. I didn't realize I didn't have this cap - bummer. I do have a 100uf 100v polarized cap though.

Can I sub this cap in as long as I put the negative side to ground?

Thoughts?




Initial test show this is working, but it seems a bit weak - probably because it is a flea watt SE amp. WOuld anyone have thoughts about uping the coupling caps from .047(original value) to .1? Would it help the amp handle the bass response?

Mark

 

[jaymanaa]

That's fine, that value is not critical and I typically go larger. It's impossible to make a single ended 6BQ5 sound bad anyhow. Impossible. Just play with it, and see what happens. What are you driving it with?

 

[Jcricket]

Hey Jay, thanks for the input.

What about the 100uf 15 volt cap. NP vs Poloarized?

Right now it is on my work bench, I am testing it with my kenwood c1 and an almost complete ewave speaker. It will get hooked to a marantz 3350 when done though.

I think I'll drive a set of leak sandwich speakers too.

 

[Jcricket]

Got my resistors and caps today.

With the exception of the feedback caps and resistors everything is now new (or checked).

My amp does something a little bizarre. The left channel does not like bass. If I play one particualr song - where the bass is really low(probably like 35hz), the channel buzzes and crackles during the bass. I tried several new 6eu7's and no effect. I tried switching outputs from right to left and it still buzzes. What is interesteing is that the left channel buzzes when I switch output tubes too. Also, the low bass rattles occurs at any volume - even really low.

Right channel works alway, and correctly, no matter wht tube is in the unit.

I am a bit befuddled?:scratch2::scratch2:

I am assuming it has to be in the feedback circuit. I don't have a schematic so I haven't done any actual voltage checks. I'll get out the RCA manual in a bit and start checking voltage parameters and see how they look.

The feedback loop consists of a 22k resistor and a 250pf cap. Does this sound reasonable for a SE el84 circuit?
So what would you check next?

BTW - what should the opt load impeadance be for a SE EL84 circuit? 4k since it is half a PP circuit?

Mark

 

[Jcricket]

another very interesting thing. This amps ouput transformers have a b+, screen, plate taps, but it also has one more tap that it gets a signal from the 6eu7.

Does anyone have an idea what kind of circuit this is?

 

[Scuzzer]

The feedback loop consists of a 22k resistor and a 250pf cap. Does this sound reasonable for a SE el84 circuit?
So what would you check next?

Who knows? The feedback resistor forms a voltage divider with the unbypassed cathode resistor of the 6eu7 (just guessing) so without the extra info it's hard to say how much feedback is being applied.

It's easy to check if you have feedback issues. Just disconnect the feedback loop and see if it solves your problem.

 

[Jcricket]

I assumed this was feedback. THis is kind of a strange critter. I am going to go sit at the kitchen table and see if I can draw out the schematic on this.

Maybe that will help.

 

[Jcricket]

Here is the schematic.
I don't have voltages measured yet. Also, this is only one channel, but the other is identical.

My problem, one channel will not tolerate any deep bass, it falls apart and almost acts like it is clipping - no matter what the volume is.


The other channel works fine.
I have replaced and checked all resistors/caps except for hte 22k on pin 4 of the 6eu7.

ANy ideas where to start looking?

Edit: I have not replaced the PS electrolytic yet either.

 

[Scuzzer]

Your schematic looks fairly standard except the 6bq5 grid resistor going to the output transformer. If that's drawn correctly I'm not really sure why it goes there since normally it would be grounded.

Have you tried the bad channel with the feedback loop disconnected?

 

[Mike Stehr]

Your schematic looks fairly standard except the 6bq5 grid resistor going to the output transformer. If that's drawn correctly I'm not really sure why it goes there since normally it would be grounded.

Have you tried the bad channel with the feedback loop disconnected?

That's what got me scratching my head as well. Maybe unhook the feedback circuit and run the 470K grid resistor to ground...

Since you are still using the old PS cap, maybe that might be the cause. You never know...

 

[Jcricket]

Hey Scuzzer,
Yep you are right the 470k is grounded. That resistor goes to a solder lug and connects to the wire that goes to the OPT. That solder lug is a ground point - I missed that.

I think I am just going to replace that 22k resistor and see what happens. I think I have some 27k or 33k to try.

 

[jaymanaa]

I would be looking at the feedback loop too. Has the small cap and resistor on it been checked? I would definitely unhook that feedback loop whereever handy (maybe at the OPT), and see what happens. If the resistor or cap proves to be the problem, I'd experiemnt with it some. Heck, you may like it with no feedback at all. I have a couple SEP amps that run very little or no feedback. Do you have a distortion analyzer, scope, and signal generator?

 

[Jcricket]

yeah JAy, I have all three. I am okay with the scope and generator, I am still a little green but I manage. On the distortion anlyzer I haven't learned how to use it yet. Maybe now is the time to learn.

I'll mess with the feedback loop and see what happens. Maybe I increased the coupling caps too much?

 

[analog addict]

Suggestions...

On my SE 6BQ5 amp with feedback, I was told to get rid of the cap in the loop, since it changed the phase of the feedback. I think 22K was the value of the resistor I used.

I think there was also a smaller resistor to ground at the cathode of the 6EU7, I'll have to look.

Lastly, the coupling caps I used were 0.22uF. Perhaps 0.47uF is a tad large?

Oh and if no one has mentioned it, the 100uF cap is part of your cathode biasing on your 6BQ5's and I believe a polar cap would work fine.

 

[Scuzzer]

yMaybe I increased the coupling caps too much?

What were the original coupling cap values?

 

[Jcricket]

originally .05 uf
10x is probably too much. I'll drop them down to .1 and see what happens.

 

[hittlevm]

Amc

Thats an AMC amp. I have one just like it, they sound very good.

 

[Thespeakerdude8]

Thats an AMC amp. I have one just like it, they sound very good.

Correct. I worked on his little AMC before he got it- recapped it and stuck a power switch in it to get it going, and another friend put banana jacks on. That little guy surprised all of us! So much so, that I made a clone of it in an old Voice Of Music chassis, power, and output transformers. Circuit is all new with Nichicon UPW filter caps and Dale/Vishay 1% metal film resistors. I used some old .1uf 600v TRW polycarbonate coupling caps that sound FANTASTIC. I also put a dual ganged pot in place of the NFB resistor so that I can dial in the amount of damping that I want for any speaker. I changed from a 6EU7 to a 12AX7 since this was a "new" build and I had to actually punch a hole for the 9 pin preamp socket on this particular chassis. 6EU7 is simply a 12AX7 with a scrambled pinout so that the input grids are wired further away from noise inducing pins. I switched simply because I had some nice 12AX7's to try. I rolled some Telefunkens, RCA's, Raytheons, Brimar, Mullard, and Amperex. The ones I stick with the most are my Telefunken ribbed plate and Amperex Bugle Boy. Raytheon black plate 12AX7's have a lot of music in them too!!

Oh yeah, and the VM amp originally had a 5Y3 instead of a 6ca4, so I wired it back up for one.

Use a 100uf cap for cathode bypass, and get rid of the NFB bypass cap. Although, I did have mine on the scope and it needed a little capacitance to keep the output transformers from ringing...but that will vary from transformer to transformer.