A really dumb question.
Have you adjusted the bias? Have you got -19 volts on test point C, if not adjust VR7 so you do.
The other thing is what AC voltage do you measure on the B+ secondary (brown to red) with no load? I would have thought you would see more than 414 volts with no load if you expect 425 volts 'loaded' as per the schematic?
Have you adjusted the bias? Have you got -19 volts on test point C, if not adjust VR7 so you do.
The other thing is what AC voltage do you measure on the B+ secondary (brown to red) with no load? I would have thought you would see more than 414 volts with no load if you expect 425 volts 'loaded' as per the schematic?
not a dumb question.. Yes Bias is set. I have it plugged into a Variac set at 240VAC with a Dimb bulb tester in series after that... it measures 414VDC after diodes then taps off into a circuit full of CC resistors that are mostly a little bit up and down. I am not looking for perfect measurements, just ball park would be nice.
Just to be sure i re shuffled 7868's again installing only one tube at a time with the same 30V voltage drop on the same socket.. i then tried a socket on the other channel with same result.
I also, even though they measured fine on my ESR meter, removed the new C50 and C49 and re installed the old caps.
Just to be sure i re shuffled 7868's again installing only one tube at a time with the same 30V voltage drop on the same socket.. i then tried a socket on the other channel with same result.
I also, even though they measured fine on my ESR meter, removed the new C50 and C49 and re installed the old caps.
with prices of caps and rectifiers on the really cheap these days,
it would be a no brainer to choose "fresh caps" in the voltage doubler psu,
and 3A, 5408 rectifiers....
these amps uses plate load resistors that look different from your typical carbon comps, they fail without giving a hint, just measure the plate voltages on the small tubes and when you find 0 plate volts, you know that the plate resistors have become open..
i repaired two of such amps, one i converted to ac 230 volts from a rating of ac 100 volt primary, and another one where one channel was not working..
it would be a no brainer to choose "fresh caps" in the voltage doubler psu,
and 3A, 5408 rectifiers....
these amps uses plate load resistors that look different from your typical carbon comps, they fail without giving a hint, just measure the plate voltages on the small tubes and when you find 0 plate volts, you know that the plate resistors have become open..
i repaired two of such amps, one i converted to ac 230 volts from a rating of ac 100 volt primary, and another one where one channel was not working..
Dumb question 2: You mention the bulb tester in circuit after the Variac a couple of times, Are you taking the B+ DC voltage measurements with the bulb in circuit or out?
Just that the idle current draw of the output valves (according to the circuit) is 144mA times the B+ volts, say 400, and therefore at least 57.6 Watts. So if the bulb is in circuit it will be sharing the 240 volts and dropping the effective AC input voltage.
Just that the idle current draw of the output valves (according to the circuit) is 144mA times the B+ volts, say 400, and therefore at least 57.6 Watts. So if the bulb is in circuit it will be sharing the 240 volts and dropping the effective AC input voltage.
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Ha.. You're right! crikey.. I think I've done about 20 laps of the circuit 
Well at least I have learnt a lot about tube circuits. Still a lot more to learn though and i still have that hiss...
I am going to order up a full set of resistors and rework some of what i have already done... making a clean job of point to point work is harder than it looks..
Well at least I have learnt a lot about tube circuits. Still a lot more to learn though and i still have that hiss...
I am going to order up a full set of resistors and rework some of what i have already done... making a clean job of point to point work is harder than it looks..
I will replace all caps mate.. someone has already been in this amp and given it a tickle at some stage.. There where some odd caps, German PINEX around the preamp and lots of resistors replaced..all clean work though looks factory..nothing was open probably due to previous repair.. its like half the jobs been done..
Just a bit of an update on this.. I managed to cure the hiss by replacing the CC grid stoppers with some carbon film resistors i had on hand.. I've been trying to make up an order of parts to finish the amp but also experimenting replacing resistors and swapping caps around. It would be nice to have ideal replacements for these Soshin silver mica caps just to make a complete restoration and i would like to replace with modern equivalents to be honest if anyone has suggestions.
The amp is running well now, but I am not satisfied with it. It is just too bright lacking bottom end.. I have re checked all my replacement capacitor values which are good. I know this amp should sound a lot better..so i guess i am just fishing for advice?
The amp is running well now, but I am not satisfied with it. It is just too bright lacking bottom end.. I have re checked all my replacement capacitor values which are good. I know this amp should sound a lot better..so i guess i am just fishing for advice?
Been a while, lock down has done me some favours as i had this amp sitting on the bench for a few years waiting for me to get back to it, I'd kind of lost interest in my audio hobby for a bit.
So, I started from scratch and worked my way through the circuit and found a few mistakes. I had a grid stopper pinned to the wrong post, and some cap values wrong.
I have spend quite a bit of time working through it now, replacing all remaining old solen caps with a mix of mica and poly, and i have taken every CC resistor out which were way out of spec and replaced with mostly carbon film.
I have also replaced voltage doubler caps, the originals reading up around 150uF and i noticed the difference when replaced with 100uF spec.
I done all my cap and resistor replacements one channel at a time and its been a real learning experience.
the amp sounds incredible now, completely in love with it! quiet as a mouse. just need to replace some rca's maybe speaker jacks, paint the case and she's done. Well..is an Amp ever finished?
So, I started from scratch and worked my way through the circuit and found a few mistakes. I had a grid stopper pinned to the wrong post, and some cap values wrong.
I have spend quite a bit of time working through it now, replacing all remaining old solen caps with a mix of mica and poly, and i have taken every CC resistor out which were way out of spec and replaced with mostly carbon film.
I have also replaced voltage doubler caps, the originals reading up around 150uF and i noticed the difference when replaced with 100uF spec.
I done all my cap and resistor replacements one channel at a time and its been a real learning experience.
the amp sounds incredible now, completely in love with it! quiet as a mouse. just need to replace some rca's maybe speaker jacks, paint the case and she's done. Well..is an Amp ever finished?
Well, I bought exactly this amplifier last year and I’ve just read you mentioned on it, and I wonder if it made sense.
The noise/trouble you wrote about probably comes from the preamplifier (V3 / V4). Let me explain.
If we study the amplifier's schematic diagram, we notice its unusual structure of the audio path (although similar as, e.g. the Sansui AU-80), i.e. section in the order (from input to output): the input selector (+ the RIAA / TAPE HEAD / MIC preamplifier), the volume control, the preamplifier with filters, tone control, balance control, and the power amplifier (bold is intentional). In this configuration, if the volume control section is at the beginning of the audio path, we can hear the preamplifier's own noise at a constant level in the speakers, even when the volume control of the amplifier is set to the minimum.
While other amplifiers of that time (e.g. Pioneer SM-83) as well as today's ones are constructed in such a way that the volume control section is located behind the preamplifier, just before the power amplifier. The order of the other sections does not matter much, in case of tubes, should only remember about the impedance, so as not to overload them too little.
Such a configuration guarantees that the preamplifier's own noise will be inaudible to the human ear, because this component is negligibly small in relation to the constant level of the audio signal, even when listening to music quietly.
I made just such that change, i.e. I replaced the balance control section with the volume control section (in addition, in the V3 / V4 preamplifier it was necessary to reduce the gain by adjusting the feedback, increase C27 / 28 and decrease R45 / 46, because the preamplifier is fed with the maximum input signal level). Before making the changes, I tested it in LT Spice, so if the amplifier is not damaged, changes described above will suffice. No changes except for dry electrolytic cap if necessary, blown resistors and faulty potentiometers.
Finally, in my opinion, amplifier sounds great.
Let me know your opinion on it please.
The noise/trouble you wrote about probably comes from the preamplifier (V3 / V4). Let me explain.
If we study the amplifier's schematic diagram, we notice its unusual structure of the audio path (although similar as, e.g. the Sansui AU-80), i.e. section in the order (from input to output): the input selector (+ the RIAA / TAPE HEAD / MIC preamplifier), the volume control, the preamplifier with filters, tone control, balance control, and the power amplifier (bold is intentional). In this configuration, if the volume control section is at the beginning of the audio path, we can hear the preamplifier's own noise at a constant level in the speakers, even when the volume control of the amplifier is set to the minimum.
While other amplifiers of that time (e.g. Pioneer SM-83) as well as today's ones are constructed in such a way that the volume control section is located behind the preamplifier, just before the power amplifier. The order of the other sections does not matter much, in case of tubes, should only remember about the impedance, so as not to overload them too little.
Such a configuration guarantees that the preamplifier's own noise will be inaudible to the human ear, because this component is negligibly small in relation to the constant level of the audio signal, even when listening to music quietly.
I made just such that change, i.e. I replaced the balance control section with the volume control section (in addition, in the V3 / V4 preamplifier it was necessary to reduce the gain by adjusting the feedback, increase C27 / 28 and decrease R45 / 46, because the preamplifier is fed with the maximum input signal level). Before making the changes, I tested it in LT Spice, so if the amplifier is not damaged, changes described above will suffice. No changes except for dry electrolytic cap if necessary, blown resistors and faulty potentiometers.
Finally, in my opinion, amplifier sounds great.
Let me know your opinion on it please.