My hybrid amp has a slight distortion in certain passages (eg piano with lots of reverb) and I have tried just about everything I can think of to linearize the output mosfets. I have also inserted a resistor between the anode and grid of the EL34’s (previously connected directly). The mods have brought about some good improvements, but the distortion is not completely gone. Now I am starting to suspect it may be a sagging heater supply. The heater voltage sits at 5.5v. Is this maybe low enough to create a little bit of distortion on complex waveforms like I am experiencing?
Provided the heater voltage is constant, that won't be the cause of the distortion. In fact, slightly low heater voltages tend to improve the linearity of valve! (well in triodes and diodes it does, not sure about pentodes).
It doesn't matter what the valve is doing, there should be no reason for the heater voltage to sag.
It doesn't matter what the valve is doing, there should be no reason for the heater voltage to sag.
Hi hihopes,
Tubes are actually designed to run at the rated heater voltage for the longest tube life. A slight reduction has been noted to reduce noise in signal tubes. Your reduction is more severe than I have seen anywhere before. Try 6 VDC for a change, then 6.2 V. I am assuming you are running DC heaters through a regulator of some kind.
-Chris
I think your heater voltage is too low. Why not try to increase it a bit?
Tubes are actually designed to run at the rated heater voltage for the longest tube life. A slight reduction has been noted to reduce noise in signal tubes. Your reduction is more severe than I have seen anywhere before. Try 6 VDC for a change, then 6.2 V. I am assuming you are running DC heaters through a regulator of some kind.
-Chris
Your amp is push-pull, right?
Your description sounds like crossover distortion; piano usually shows this more than most sounds.
Does it sound like 'fuzziness' surrounding the notes? That's my description, anyway.
Low heater voltage would cause clipping of peaks. 5.5 volts is more than 10 percent low; probably should be fixed anyway.
Your description sounds like crossover distortion; piano usually shows this more than most sounds.
Does it sound like 'fuzziness' surrounding the notes? That's my description, anyway.
Low heater voltage would cause clipping of peaks. 5.5 volts is more than 10 percent low; probably should be fixed anyway.
Hi diy4, your description is pretty accurate. Most of the time it is crystal clear, but on one particular track with piano and lots of reverb, there is this fuzzy halo that is more pronounced on some notes than on others. For the rest of the CD it can't be heard, but I know it isn't the CD because my other amp, a Hart Linsley Hood kit amp (with the same output mosfets) doesn't reproduce the same effect. I have done all I can think of to clear up the mosfets and I have changed the input transistors, so that is why I started to suspect the valves.
I tried to power the heaters with a regulated supply, but in my zealousness, i damaged a pair of EL34's (must be too much current because the voltage never rose above 6.1v) I still don't know why it happened.
I tried to power the heaters with a regulated supply, but in my zealousness, i damaged a pair of EL34's (must be too much current because the voltage never rose above 6.1v) I still don't know why it happened.
Hello anatech. Thanks for the input. To answer your questions, no, the supply is AC, from a transformer that probably started out life as a 6v, but providing a few more fractions of a volt than it is doing now. I was concerned about the low voltage and thought i would do the valves a favour and provide them with a regulated supply. I had a 12v 5 amp supply lying around, so I worked out the resistor values required for LM317 to supply 6.25v and implemented it. I figured that the job was going to be too much for a single LM317, and probably even 2 would overheat, so LM317's being cheap, i built it with 4. when I connected it to the valves, it never rose above 6.1v, but soon started to drop fast. I switched it off, but by then the damage was done.
Do you think there is a way to use that 12v PSU, or will I have to start from scratch with another heater supply?
Do you think there is a way to use that 12v PSU, or will I have to start from scratch with another heater supply?
Crossover (or 'notch' ) distortion happens when the output stage bias is incorrect. A 'dead area' results where both output devices are turned off. This could be a design flaw, or incorrect bias. Can you post a schematic?
The EL34 heaters are not the cause; you should raise the heater voltage a bit, but they are probably working ok, almost.
The EL34 heaters are not the cause; you should raise the heater voltage a bit, but they are probably working ok, almost.
diy4 said:
Not from this typical output stage; IMO it isn’t worth operating power tubes at lower heater voltages. Tests on a 100W p-p AB1 amp using 4x 6550B’s gave markedly worse performance. First figures = heaters.
6.35VAC; Pout =80W; 1Khz; Thd = 0.05%; B+450V; Iq = 85mA each tube, is corect operation.
5.7VAC: Pout = 70W; 1Khz; Thd = 0.15%; B+ 450V; Iq = drops to 73mA each tube.
The clipping level is correspondingly lower with a sharp rise in thd.
At higher frequencies this would be far worse. I can't measure IM thd but the results would be poor.
Generally I find the 6550A,B,C series is heater / emission sensitive, and probably not the only tube in the power class to be so.
Verdict; For optimum power throughput performance use correct heater voltage or a touch higher. I use 6.35V.
richj
Hi rich,
How many people do you think read tube manuals these days?
Hi hihopes,
If you don't have a tube manual, buy one. The RCA manuals are good as an example.
Understand that EL-34's can draw up to 1.6 amps each. Make sure your filament transformer is rated 1.25 X your total load or higher. Continue using AC for your heaters. You can drop your voltage a couple ways. You can use a series dropping resistor, or you can pre-regulate the AC going into the transformer. This is much more complicated.
-Chris
How many people do you think read tube manuals these days?
Hi hihopes,
If you don't have a tube manual, buy one. The RCA manuals are good as an example.
Understand that EL-34's can draw up to 1.6 amps each. Make sure your filament transformer is rated 1.25 X your total load or higher. Continue using AC for your heaters. You can drop your voltage a couple ways. You can use a series dropping resistor, or you can pre-regulate the AC going into the transformer. This is much more complicated.
The inrush current was probably the final blow to these regulators. You are dropping too much voltage. Even one EL34 is too much for an IC regulator if it's one of the 1.6 amp beasts. Dropping 10 VDC or more across those regulators was most unkind to them. If you are fixed on this course, use a high current switching regulator for each channel. A "soft start" would be an excellent idea. Look into the LM2677 switching regulators for your project.
-Chris
anatech said:Hi rich,
How many people do you think read tube manuals these days?
-Chris
Hang on .. Measuring NOS GE 6550's I got 1.3A heater current at 6.3V..(def on the low side and yes I measured it correctly). Another wild upwards variation is with the KT90 where I got 2A. (That's more like it).
The RCA tube manual for the 6550 quotes 1.6A at 6.3V, so what's up ?
Sounds like I've been given out of spec tubes, but my supplier says Nup. >Checking with another = same reply.
Simply have to accept.
richj
Hi rich,
Well, that's the thing. Nominal specs should only apply to one brand. The type registration should list the maximum heater current allowed (in a perfect world). Types like 12AX7 heater currents should always be bang on.
If I look up 6550's in GE and RCA, I get (GE) 6.3V 1.8A and (RCA) no listing as of RC-27. A Genalex lists the KT88 as 6.3V and 1.6 amperes also.
I expect there may be some differences in tubes intended for parallel heater operation simply because the cathode coating recipe used by some manufacturers may require a little more current to emit the required amount of current. Your GE's may be late production which allowed them to drop the heater temperature a little. This would be a good thing as far as reliability is concerned.
I am looking at a matched quad of Westinghouse 6550A's in their original boxes. All broken when I moved.
I used to have a Westinghouse tube manual (still at old shop). Anyone know where I can get a copy? I would have looked up their spec too. I'll bet it's 1.6 amperes.
-Chris
Well, that's the thing. Nominal specs should only apply to one brand. The type registration should list the maximum heater current allowed (in a perfect world). Types like 12AX7 heater currents should always be bang on.
Well, a KT90 isn't exactly a 6550A. A KT88 should be closer - no?
If I look up 6550's in GE and RCA, I get (GE) 6.3V 1.8A and (RCA) no listing as of RC-27. A Genalex lists the KT88 as 6.3V and 1.6 amperes also.
I expect there may be some differences in tubes intended for parallel heater operation simply because the cathode coating recipe used by some manufacturers may require a little more current to emit the required amount of current. Your GE's may be late production which allowed them to drop the heater temperature a little. This would be a good thing as far as reliability is concerned.
I am looking at a matched quad of Westinghouse 6550A's in their original boxes. All broken when I moved.
I used to have a Westinghouse tube manual (still at old shop). Anyone know where I can get a copy? I would have looked up their spec too. I'll bet it's 1.6 amperes.
-Chris
richwalters said:
If you read the original post by hihopes you may notice that his amp uses Mosfets, not tubes, as the output devices. The EL34's are voltage amplification stages wired as triodes.
As you noted, an AB output stage will clip sooner and have more distortion when heater voltage is low; however, his description of the problem he is having is not that .
You are talking about a tube output stage; his is solid state.
Your scenario involes clipping, his problem is crossover distortion.
As I noted twice in earlier posts, the EL34 heater voltage is low,
and should be corrected. It is not the cause of his problem, however.
Hi diy4,
With low emission, he may be having trouble with the mosfet gate charge issue. It is entirely possible that heater temperature may make this less of a problem.
One thing I can not understand is this. Why use an EL34 when a 6L6 type has much less heater current demands? There are other tubes that would probably suit this use even better. An EL34 would be my last choice here.
-Chris
Edit:
Hi Rich,
I know that. It's wise to fix known problems as you find them. Amazingly, other faults tend to disappear sometimes.
With low emission, he may be having trouble with the mosfet gate charge issue. It is entirely possible that heater temperature may make this less of a problem.
One thing I can not understand is this. Why use an EL34 when a 6L6 type has much less heater current demands? There are other tubes that would probably suit this use even better. An EL34 would be my last choice here.
You are certainly not the only one who has noticed this.
-Chris
Edit:
Hi Rich,
With any luck! The Electroharmonix brand has been very good this way. May they continue to bring us good tubes at good prices!
Thanks for all the input guys. Sorry, I would love to get hold of a full schematic myself. (The guy who designed and built the amp gave me one about 10years ago when i bought the amp, but my overzealous wife must have "tidied" it away into file no.13. I have asked him for a copy on a few occasions, but he is pissd that I lost the one he gave me. He is overseas for another month or so. What is it that you want to look at? Can I describe something for you?
I think I need to clarify that the problem I am trying to address is not very obvious, nor is it very often apparent. The information that has been provided by so many here seems to confirm that it is most probably the mosfets that are ringing/oscillating under very particular circumstances only.
BTW anatech, the LM317's are fine. 4 x Lm317's for a total of 3amps @ 6v drop should not have been a problem according to my calculations. (Of course, i could always be proved wrong) It was the heaters of the EL34's that suffered. Maybe they drew too much current when they were cold? (I guess 4 x LM317's should be able to supply 6A minimum)
BTW anatech, the LM317's are fine. 4 x Lm317's for a total of 3amps @ 6v drop should not have been a problem according to my calculations. (Of course, i could always be proved wrong) It was the heaters of the EL34's that suffered. Maybe they drew too much current when they were cold? (I guess 4 x LM317's should be able to supply 6A minimum)
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