Remember These?

What people are working on at the moment
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jack
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#631 Re: Remember These?

Post by jack »

Btw. I use a dab of "hot glue" to keep them in position.
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Mike H
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#632 Re: Remember These?

Post by Mike H »

I-hate-being-right-all-the-time.jpg
I-hate-being-right-all-the-time.jpg (26.82 KiB) Viewed 6933 times
Image

Seriously tho I'm really pleased for you because it's a terrific amp, and I well remember the last OTL you brought to Owston and which looked fab but oscillated lkke bu55ery all over the place and you were quite peed off.

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#633 Re: Remember These?

Post by Cressy Snr »

Cheers Mike.

Cracking my personal “curse of the OTL” is I feel, the best thing I’ve done as regards DIY amp building.
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#634 Re: Remember These?

Post by Mike H »

Good feeling innit? Image
 
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#635 Re: Remember These?

Post by IslandPink »

Top notch !
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#636 Re: Remember These?

Post by Wolfgang »

I think this is a very good compilation from R.Coleman in diyaudio on the topic of "anode stopper, grid stopper, ferrit beads":



"Oscillations in a triode (or MOSFET), at high frequency occur though unintentional feedback paths, especially from anode circuit to grid circuit. The 180 degree phase relationship between these two mean that you only need to find a frequency where the effective coupling between the two electrodes gives an effective gain of 1. That satisfies perfectly the operating condition for an oscillator.

To get a gain of 1 between anode & grid you need sufficient electromagnetic coupling between the two, and a fairly high impedance at the grid, or gate (MOSFET), base (BJT).

A grid stopper works by presenting a lowpass filter at the "oscillator" input, reducing the effective gain at potential oscillation frequencies. It's an RC filter formed with the Miller-boosted capacitance of the grid. It is vital to have the stopper very close to the input pins, or else the electromagnetic fields can couple to the grid side of the stopper, bypassing the filtering effect.

A stopper in the anode circuit effectively prevents "transmission" of electromagnetic fields at high frequencies. It forms an RC lowpass filter with the anode capacitance (looking inward) and with circuit capacitance (looking outward) - an unbalanced pi-filter.

The cathode circuit is very low impedance compared to the grid, and so its susceptibility as an HF receiver is very poor. And because its voltage swings are usually much lower than the anode's, it is much less effective as a transmitter too.

It's possible to contrive circuit conditions where a cathode stopper would be effective, but for most configurations, it's near to useless. A grid stopper is almost always the correct way to go, you just have to implement it correctly:

- mounted so as to present the MINIMUM amount of conductor attached to the grid pin. The shorter the effective length, the weaker the coupling will be.

- Correct value: It's hard to judge the effective gain at oscillation frequency, and hence the degree of Miller effect, but aiming to produce a time constant of 5 to 10us will usually be safe for audio, based on the datasheet value for input capacitance.

- Construction. Avoidance of inductive parts is worthwhile - even low values of L will form a series-resonant LC at some frequency, and U/VHF triodes like ECC88 may well amplify at that wavelength. So SY is right to adopt carbon composition, as these are very low inductance. 1206 chip resistors also work well.

- Stubborn cases can often be suppressed even better using chip ferrite parts such as the MuRata BLM21A102S or its near relatives. These are not inductors, they are frequency-dependent resistors, and possess very useful cutoff characteristic. Just be careful to mount them rigidly, eg on a piece of FR4, as they may generate noise under vibration."




I don't use anode stoppers but the 10k grid stoppers were absolutely necessary to prevent some kind of oscillation in the 6C33 OTL. Never had any problems with these amps of any kind (now about 1000hrs of use). They behave like the most reliable and unproblamtic OTLs I have. And they still sound absolutely over the top.
But I was wondering about the pros and cons of of carbon compound resistors at the grid. Obviously they are low inductance but also must add some extra noise which is highly controversial at the grid position. Are these resistors noisier ?


There might be also a serious competitor for the single tube OTLs: The Inverted OTL. I am building one with 6SN7/6AS7 tubes, reg. B+, six output tubes per channel ( Zout=9,4R) and a first listening (only one channel in my shop) showed me the areas that still can be improved dramatically in the case of OTLs.
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#637 Re: Remember These?

Post by Cressy Snr »

Hi Wolfgang,
Nice to see you on here again.
Carbon comp grid stoppers are noisier but I don’t think they’d cause much of a problem on an output valve.

It’d be nice to see your inverted OTL on this forum. I for one would be interested in seeing one built.

The 13E1 OTL I’ve built is giving no problems at all and sounds wonderful. I love it. :)
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#638 Re: Remember These?

Post by Wolfgang »

I will start a new thread about this amp soon, Steve. Even if nobody on this forum will build it because the ratio of power demand vs output power is even worse than with typical SE OTLs -and from many comments I got the impression that OTLs are anyway not highly coveted here -it will be at least entertaining. So let's see how this will develop!
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#639 Re: Remember These?

Post by Cressy Snr »

Wolfgang wrote: Sun Jun 09, 2019 3:29 pm I will start a new thread about this amp soon, Steve. Even if nobody on this forum will build it because the ratio of power demand vs output power is even worse than with typical SE OTLs -and from many comments I got the impression that OTLs are anyway not highly coveted here -it will be at least entertaining. So let's see how this will develop!
I think that there is plenty of interest in OTLs here. But as a community, we are a pretty broad church, so there is all sorts of other stuff going on. My 13E1 amp seemed to go down well at the Owston audio meet last week. Whether it’d generate any other builds from anyone else is hard to say.
SEOTLs do require 100dB/W speakers; preferably of the steady impedance, low reactivity, single-driver persuasion and they have largely gone out of favor on here over the years; superseded by multiple driver open baffles or small 4 driver plus tweeter line arrays.
Even the single coaxial driver MLTLs Colin and Scott have recently come up with would be hard pushed to go much above a whisper with a SEOTL amp.

But it’ll be great to follow your inverted OTL thread. I’m certainly looking forward to it. :)
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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Paul Barker
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#640 Re: Remember These?

Post by Paul Barker »

Wolfgang wrote: Sun Jun 09, 2019 3:29 pm I got the impression that OTLs are anyway not highly coveted here -it will be at least entertaining. So let's see how this will develop!
Wrong impression
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#641 Re: Remember These?

Post by Cressy Snr »

Because these tubes are a bit more expensive than 6C33C, I’ve been doing some careful tailoring of the current sinks. Tweaked the sense resistors with an extra 0.15R. This has brought the plate dissipation down to 65W - conservative for a six cooling fin 13E1, but should get the heat down, both from the valves and from the LM317HVT current regs.

The main reason I’ve done this, is see if I can accommodate the two 1950s single cooling fin equipped 13E1s I have. Both get a bit on the dull red side at 75W. I haven’t tried them as yet because it’s not dark enough. I’ll see if the old valves can refrain from getting hot and bothered with the lower current, later on this evening.

I fancy that with the lower current, the higher frequency instrumentation has become just a little quicker on the leading edge, with more air and sparkle, but I could be talking nonsense as is my wont. Let’s say that there have been no ill-effects on the presentation and be content with that for now.
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#642 Re: Remember These?

Post by Cressy Snr »

Tried the 1950s single cooling fin valves with the lower current.

One is spot on, the other still has a dull red centimetre square hot spot on one side at the very bottom outer corner of the plate structure.
I think the offending tube is one of those renegade types that Morgan Jones observed to “just get hot and bothered, no matter what current they were run at”
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Ray P
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#643 Re: Remember These?

Post by Ray P »

Steve, did you think any more on running DC filaments on the 13E1s.

I've got a Mean Well SMPS with a nominal rating of 24V 4A that I was thinking of trying; the output voltage can be tweaked a little to a maximum of around 28V so pretty much ideal for the 13E1s. The filament supplies shouldn't be quite as critical as with DHTs so using SMPS should be a cheap and cheerful solution - unless anyone knows better?
Sorry, I couldn't resist!
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#644 Re: Remember These?

Post by Cressy Snr »

Ray P wrote: Wed Jun 12, 2019 10:02 pm Steve, did you think any more on running DC filaments on the 13E1s.

I've got a Mean Well SMPS with a nominal rating of 24V 4A that I was thinking of trying; the output voltage can be tweaked a little to a maximum of around 28V so pretty much ideal for the 13E1s. The filament supplies shouldn't be quite as critical as with DHTs so using SMPS should be a cheap and cheerful solution - unless anyone knows better?
Hi Ray,
I’ve not gone any further on the DC heating question. There’s no room in the chassis for DC supplies. I’d have to rebuild as monoblocks, in order to be able to fit the supplies in. Each mono would have to be as nearly big as the stereo chassis I currently have, so at the moment, it’s not really a realistic proposition for me.
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#645 Re: Remember These?

Post by Ray P »

Yes, space is always a limiting factor in a domestically acceptable project. I only ask because you mentioned it at Owston - as an experiment you could hang some wires out of the bottom of the chassis to a remote SMPS but that becomes a dangerous game if you like the result, sometimes ignorance is bliss!
Sorry, I couldn't resist!
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