Remember These?

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Wolfgang
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#181 Re: Remember These?

Post by Wolfgang »

Cressy Snr wrote: Sun Dec 09, 2018 10:36 pm ... it has taken a bit of fine adjustment of feedback, tube dissipation and B+ values in order to optimize the amplifier for maximum output with the least distortion into my own particular speakers, and helpfully, optimizing the amp has not actually required giving the output tubes a thrashing. It needs a sensitive approach,.....
This is absolutely true. And i think in the meantime even more so with these kind of single output tube OTLs as there is quite a bit of current through the tubes.
I finally installed my prototype protection circuit for the 6C33 over the weekend and had to replace the 2R7 resistors with 10R/2W pots for setting the current and the trigger point for the circuit so that it will shut down the tubes in case of a fault. And, boy, did this pot change the sound again! Fortunately to the better and not to the worse but still this couldn't have been expected. When I replaced the same resistors with stepped attenuators for fine tuning Ip for each 300B in my SE OTL there was no audible difference regarding SQ. Only the "nervousness" was suddenly gone. Resistors and pots are both wirewound.

Another good reason for the pots instead of resistors is that the tubes get weaker over time (the 6C33 dropped 20mA after about 500hrs of use) and can be easily readjusted to the ideal value.
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#182 Re: Remember These?

Post by Cressy Snr »

Hee hee what larks!

I know, let's see if that dodgy 13E1 from 10 years ago was really as dodgy as I remembered.
Insert into valve socket, fire her up and BANG! lovely purple firestorm, deafening crackle from the associated speaker and one CCS chip no longer working properly. Oh poo I said.

Yep your memory hadn't failed Steve; that valve really was as dodgy as you thought.

Another chip is currently epoxied to the heatsink and I'm waiting for the glue to go off. Good job I bought half a dozen TL783s.
The offending 13E1 is now in the bin. Aren't breadboards fun. :roll:
Last edited by Cressy Snr on Tue Dec 11, 2018 1:25 pm, edited 1 time in total.
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Mike H
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#183 Re: Remember These?

Post by Mike H »

Consign it to recycling depot ASAP. Or at least write on the box "SCRAP DO NOT USE" .. :D
 
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#184 Re: Remember These?

Post by Cressy Snr »

Them chip regs don't like it up 'em do they Mr Mainwaring? :lol:
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#185 Re: Remember These?

Post by Cressy Snr »

Now that I've committed to putting this amp in a box, is there any way to protect a reg chip from a rogue tube?

300B arcs over in the OTL I built for Nigel....it takes out the reg chip.
13E1 arcs over in my own amp.....it takes out the reg chip.

Seems to be that a tube connected to something solid state and said tube suddenly going off on one will not turn out well for the silicon bit.
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Wolfgang
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#186 Re: Remember These?

Post by Wolfgang »

Rozenblit uses zener diodes around the regulator for this reason. But they start conducting long before their break up values and it's audible. The TL783 is a HV reg. and should do of course much better than an LM317.

If "arcing over" comes with sudden increase in current through the tube my protection circuit would shut the tube off before damaging anything (10ms enough?). Didn't try this part in my series of tests. Never saw it happen in my amps. So what exactly happens in this case regarding current?

Another way of getting the safety of zeners without the disadvantage of leakage currents all the time would be to simulate the fucntion of a zener with discrete parts, building it with a transistor and diodes etc.
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#187 Re: Remember These?

Post by Cressy Snr »

Wolfgang wrote: Tue Dec 11, 2018 4:23 pm Rozenblit uses zener diodes around the regulator for this reason. But they start conducting long before their break up values and it's audible. The TL783 is a HV reg. and should do of course much better than an LM317.

If "arcing over" comes with sudden increase in current through the tube my protection circuit would shut the tube off before damaging anything (10ms enough?). Didn't try this part in my series of tests. Never saw it happen in my amps. So what exactly happens in this case regarding current?

Another way of getting the safety of zeners without the disadvantage of leakage currents all the time would be to simulate the fucntion of a zener with discrete parts, building it with a transistor and diodes etc.
Thanks Wolfgang,
Seems I spoke a little soon. It turns out that it wasn't only the regulator package that was damaged, it was also the output capacitor on the offending channel that got damaged by the surge. It was leaky, almost a short circuit in fact. It only measured 7 ohms pos to neg. Once I had replaced the reg and fired her up again the same fault was aparrent; only 7.5V on the cathode.

I found the cap fault eventually, with continuity tester and a process of elimination. The amp was only up for a few seconds then switched off immediately I saw the bias only come up to a 7.5V value after replacing the reg. This was fortunate as the DC on the speakers caused by the leaky cap could have burned out the voice coil if it had been left for any length of time.

I replaced both output caps as a matter of course.

I think you have to assume that a bad tube will take out both the CCS chip and the output cap, unless your fast acting protection circuit can intervene fast enough. I don't know whether 10ms would do it as I don't have experience of designing protection circuits.

Anyway no real harm done and she's back up and running again. It's no more of a nasty situation than a bad tube in a big push pull amp, taking out an output transformer. A lot cheaper to repair a SE-OTL though. :)
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Wolfgang
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#188 Re: Remember These?

Post by Wolfgang »

This is of course a serious problem with these kind of amps.

Two reasons come to mind as given in these two articles: gassy tube, old and not being used for many years. Reconditioning might help.

https://pa0fri.home.xs4all.nl/Lineairs/ ... ioning.htm

Solid state rectifiers in the B+ psu without preheating the tube. I am not sure about this but maybe some more knowledgeable forum member can comment.

http://www.gammaelectronics.xyz/ax_2004 ... e-arc.html

They don't recommend reusing tubes that have arced.

The heaters of the 6C33 are warming up so slowly that it is like using a delayed B+ psu. Cathode stripping at this low volatge is definitely not a problem that could harm the tube over time.

But there is one more thing which I observed while testing the circuit: If the tube is fully "open" there is arcing between two relay contacts (about 3mm apart) if the circuit shuts off the B+ connection to the tube. The same could of course happen if the tube is physically damaged and anode has such a close or even closer distance to the cathode out of some reason. For the protection circuit there is an easy solution to this problem but not for a faulty tube.
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#189 Re: Remember These?

Post by Cressy Snr »

OK, the first caps I used as replacement output caps, were a pair of 2200uF 200V Panasonics.
After a couple of hours I had to conclude that they were complete crap!
The music was shut in, flat, boring as hell, lumpy in the bass and completely uninspiring.
Bypassing them with 2.2uFs didn't help a lot.

Eventually I found a pair of 6800uF, 85V Samwaha "For Audio" caps that had once been used in my NVA clone amplifier power supply.
Image
These have brought back the magic, and even unbypassed, they knock the Panasonics into a cocked hat.
Thank Gawd for that as they say.

These kinds of amps change their sound when components are changed, much more so than any other amp I've built.
That can either be a blessing or a curse. On the one hand they are easy to tweak, but on the other it is frighteningly easy to lose the magic and the fear is, you lose it and then can't get it back.
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#190 Re: Remember These?

Post by jack »

Need to read this thread: I still have a box of 13E1s that were destined many years ago for a "Crystal Palace" amp. 2 are in poor shape (heavily silvered) but the others look good.

I've never tested these as the heater requirements just were huge (around 30W ISTR) and I just didn't have anything that would handle that plus the HT.

Something else for the project list on return to Blightly - watching with interest...
Last edited by jack on Wed Dec 12, 2018 6:41 am, edited 1 time in total.
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Ray P
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#191 Re: Remember These?

Post by Ray P »

Don't forget that the different values of the output caps you're experimenting with will be changing the -3dB rolloff point at the bottom end, which might be affecting the results you're hearing.
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#192 Re: Remember These?

Post by Cressy Snr »

Ray P wrote: Wed Dec 12, 2018 5:56 am Don't forget that the different values of the output caps you're experimenting with will be changing the -3dB rolloff point at the bottom end, which might be affecting the results you're hearing.
Oh yes, I was well aware of that, which is why I was dubious about the 2200uF in the output stage. The output stage rolloff at 2200 into 8 Ohms is around 9Hz which I believe was causing the bass hump.

The 6800uF puts the f-3dB point at about 3Hz and seems to have taken the LF resonance below where it can cause any problems. Bruce Rozenblit used two small parallel 3300uF caps on each output of the Mini Beast; probably because they were cheaper than the one large 6800uF I'm using.

I know why I didn't pick up on this. My amp was influenced by the Broskie influenced schematic yourself and Wolfgang worked out. The fact that yours is optimized for 15 Ohm Lowthers, unfortunately did not register until yesterday.

With a 15 Ohm driver the caps only need to be half the capacitance. 3300 would give about a 3Hz cutoff in your amp and in mine about 6Hz

I got the calculator out last night to try to quantify what was going on re the bottom end with the 2200s in place, and it did confirm the subjective results and reinforced the decision to get them the hell out of there.
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#193 Re: Remember These?

Post by IslandPink »

Is it something related to the feedback that would cause a resonance or bass hump with the higher roll-off ? Can you remind me which page it is for the circuit?
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#194 Re: Remember These?

Post by Cressy Snr »

IslandPink wrote: Wed Dec 12, 2018 10:17 am Is it something related to the feedback that would cause a resonance or bass hump with the higher roll-off ? Can you remind me which page it is for the circuit?
Yes, that's what I thought: the conjunction of the feedback and the cutoff resonating, and just so happening to coincide with the -3dB cutoff of the speaker cab alignment itself. With this all conspiring together the bottom end would have become a bit wayward, and it did.

Image

As you can see, there are no output cap values or Rfb values on the schematic, so I had thought of the need to optimise the values and 3300uF was a good value to use in the first instance. The 2200uF were all I could put my hands on after one of the existing 3300s were damaged, but luckily I remembered I had two extra PSU caps I'd bought for the NVA clone, these put the resonances well down, where they could hopefully, not introduce phase related problems in the bass, when interacting with the feedback. A scope would be needed to really check it out and that is on the to-do list.
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Nick
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#195 Re: Remember These?

Post by Nick »

You could try taking the feedback from before the output cap and adding a blocking cap in the feedback loop (can be much smaller). Stops the feedback from working to correct the output cap low pass filter.
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