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Fresh EL84 integrated SET build - gain issues...

perkri

Member
2017-12-05 7:59 pm
Hi.

I bought a Heathkit SA-3 integrated amp about a year ago, and after doing a rebuild - new caps/resistors - couldn't get it to work.

So, I tore it down completely, made a new "enclosure" and built a new amp from the "bits"

Powered it up safely, and no blue smoke or magic sparkles.

Sounds very nice with a couple of caveats...

The Volume knob is insanely sensitive. Reaches full volume at about 8/9 o'clock, and if it gets dialled up further, there is no change in volume. It also starts to distort quite a bit before it reaches max volume - gets grainy and "scuzzy" (Max volume is not very loud). Granted, I realize this is a flea watt amp, but I'm running it into 90db speakers and feel it should be louder overall?

I removed both the treble (C10 & C11/R13 & R14) and bass control (C17 & C18/R29 & R30) as I didn't want them and wondering if this might have something to do with it?

Also, there is a small cap (C1 & C2) as well as a resistor (R1 and R2) on the RCA inputs. Wondering what they do? Are they a filter of some sort?

Thank you!

Per
 

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The volume control should be a Linear type, not Log.
If you remove the "bass" control, you will remove the nfb loop. The distortion and gain will be uncontrolable.
C1/2 are HF filters in an attempt to stop interference arriving on the preamp input.
R1/2 stop the output from a tuner from storing a DC voltage and causing a click/pop sound when selecting it from the other selection.
 

Depanatoru

Member
2006-07-06 10:27 am
The bass control pot + cap is also part of the negative feedback loop , so it was unwise to remove them completely . Those parts should be shunted/replaced with wires . Now without negative feedback the amplification is at maximum and probably overall performance is also not very good .

The input caps to ground are for phono , for optimum response from some old ceramic cartridges ... you can remove them
 
Last edited:

perkri

Member
2017-12-05 7:59 pm
The volume control should be a Linear type, not Log.
If you remove the "bass" control, you will remove the nfb loop. The distortion and gain will be uncontrolable.
C1/2 are HF filters in an attempt to stop interference arriving on the preamp input.
R1/2 stop the output from a tuner from storing a DC voltage and causing a click/pop sound when selecting it from the other selection.
Thank you!

While I have no idea how a negative feedback loop works, what you say makes sense :)

And I’m using the original volume pot from the Heathkit, but intend on replacing it with a new “proper” one. Figured I would get the thing working properly before I bought any more parts.
 

perkri

Member
2017-12-05 7:59 pm
The bass control pot + cap is also part of the negative feedback loop , so it was unwise to remove them completely . Those parts should be shunted/replaced with wires . Now without negative feedback the amplification is at maximum and probably overall performance is also not very good .

The input caps to ground are for phono , for optimum response from some old ceramic cartridges ... you can remove them
Thank you!

Happy to not put them back!

As far as returning the bass control/negative feedback into the circuit with wires/fixed value components, where do I start at selecting values and how should I implement the jumper that goes from the left to the right?

This is where the amp currently sits from a wiring standpoint…
 

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perkri

Member
2017-12-05 7:59 pm
I would use a just a wire there , and of course the cap must be eliminated too . Like in the schematic from post #5
Thanks!

That’s what I have hooked up currently, just the wire? And the distortion/grain/volume issue is present with the wire connected. There is a dotted “jumper” going from the left to the right. Any thoughts on what to do about that?

Per
 

Depanatoru

Member
2006-07-06 10:27 am
The dotted line means that the pot is dual ...
If the gain is too high you can replace the resistor R24 27K ( and the equivalent on the other channel ) with a lower one , lets say 10K . More negative feedback , less gain .
The root cause is that it has 2 stages of amplification before the power tube , usually 1 is enough for a single ended like this .
It was made for turntable phono input so it is very sensitive , maybe 100mV rms or even less for full power .
 

perkri

Member
2017-12-05 7:59 pm
Thanks again!

I think I understand… I gather ceramic cartridges have a higher output than MM?

Would putting C1/C2 & R1/R2 help to reduce the line sensitivity?

If I were to put the bass control back in - and have it “fixed” as I’ve drawn, would that have any effect on the negative feedback?

Please forgive all the questions - trying to learn as much as possible as I go along :)

Per
 

Depanatoru

Member
2006-07-06 10:27 am
Thanks again!

I think I understand… I gather ceramic cartridges have a higher output than MM?

Would putting C1/C2 & R1/R2 help to reduce the line sensitivity?

If I were to put the bass control back in - and have it “fixed” as I’ve drawn, would that have any effect on the negative feedback?

Please forgive all the questions - trying to learn as much as possible as I go along :)

Per
Yes ceramic cartridge has much more output than MM .
C1 C2 R1 R2 don't change the input sensitivity
Even if you put back bass control the sensitivity would be about the same ( in fact probably higher ) .
Maybe the treble circuit would have some effect in reducing the sensitivity but not dramatic
I see two option , increase the negative feedback as I said , or remove the input pentode stage and feed the signal directly in the triode .
 

perkri

Member
2017-12-05 7:59 pm
Woo hoo!!!

I found the problem. I had the input/output of the volume pot wired wrong.

The center had the signal comIng in, the side had the signal going out.

Now it is as it should be.

Feeling a bit dumb…

Sorry to have wasted your time, but thank you for your thoughts, as I doubt I would have found this without digging through the other issues you pointed out.

So happy!

This sounds so good now.

Thank you!

Per