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Old 14th October 2010, 11:36 PM   #11
cnpope is offline cnpope  United States
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Quote:
Originally Posted by Guiseppe View Post

I have. HT3 is at 430v - not huge. Schematic may need proofing, though - Mellow states in the text that R29 limits the current through the output stage and speakers in the event of a fault that forces the output stage to latch either up or down; looking at the schematic, I don't see how R29 could do that. Also, the schem shows -50v to the grid of one 6C33 (V4), and -200v to the grid of the other 6C33 (V5) - is this correct?

What am I not seeing or understanding here?
R29 is a typo for R33. The -50V and -200V on the grids of V4 and V5 are perfectly correct. The cathode of V4 is at 0V, whilst the cathode of V5 is at -150V, so each of them therefore has a grid-cathode voltage of -50V.

I've built this amp, and I'm very pleased with its performance. I used a somewhat bigger chassis than Tim Mellow did, and I included a quiet cooling fan and made a few holes in the top of the chassis. It runs a bit warm, but nothing too excessive I think.

Chris
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Old 15th October 2010, 12:11 AM   #12
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Chris:

Thanks. Did you make any modifications, or build it as designed? Any issues/problems/concerns?

Joe
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Old 15th October 2010, 03:03 AM   #13
cnpope is offline cnpope  United States
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Quote:
Originally Posted by Guiseppe View Post
Chris:

Thanks. Did you make any modifications, or build it as designed? Any issues/problems/concerns?

Joe
I more or less just followed Tim Mellow's design. I didn't bother with the ammeters; I just put 0.1Ohm resistors instead, with tip jacks so I could check the current flow with an external DVM occasionally. I'm just using 200V electrolytics in the HT2 and HT4 output-tube supplies (two 2,200mfd
in parallel for each supply), rather then the string of 4 lower-voltage capacitors in Tim Mellow's PS.

I'm not using any start-up HT delay, and I checked before first running it that the EF86 pentodes V2 and V3 heat up and start functioning long before the 6C33C output tubes begin to conduct. This is important because with those 100K resistors from grid to anode on the 6C33C's, if the EF86's aren't running properly first, something quite unpleasant could happen! But anyway, everything works just fine. When first testing, I recommend using 8Ohms power resistors in place of speakers, for a first check that voltages are OK.

My speakers are Lowther DX3 with about 100dB sensitivity, and the amp can drive them much louder than I'd ever want to play them. Probably 90bB speakers will be fine too, I would guess. By the way, the hum level is so low its inaudible, even with 100dB speakers.

djn: I would have thought tube rectification would be quite a challenge. You really want the output tubes' HT supplies to be able to give two or three amps without significant drop in the voltage. Also, with HT3 needing to be negative (-430V), it all sounds a bit of a bother. Rectification is one place where I prefer to make life simple!

Chris
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Old 15th October 2010, 11:41 PM   #14
djn is offline djn  United States
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It was just a thought Chris. I have a gagle of rec tubes laying around. At this point, I don't really have the brain power to go off the reservation on this one.
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Old 16th October 2010, 01:27 AM   #15
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Originally Posted by cnpope View Post
R29 is a typo for R33. The -50V and -200V on the grids of V4 and V5 are perfectly correct. The cathode of V4 is at 0V, whilst the cathode of V5 is at -150V, so each of them therefore has a grid-cathode voltage of -50V.

I've built this amp, and I'm very pleased with its performance. I used a somewhat bigger chassis than Tim Mellow did, and I included a quiet cooling fan and made a few holes in the top of the chassis. It runs a bit warm, but nothing too excessive I think.

Chris
Chris is correct on the neg voltages. One issue I have is that the 6c33 data sheets i have all show different socket pinout numbers then the ones in the article. Can someone confirm i have attached a pdf datasheet for the tube.
Attached Files
File Type: pdf 6S33S.pdf (104.2 KB, 510 views)
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Old 16th October 2010, 04:40 AM   #16
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Quote:
Originally Posted by desperateaudio View Post
Chris is correct on the neg voltages. One issue I have is that the 6c33 data sheets i have all show different socket pinout numbers then the ones in the article. Can someone confirm i have attached a pdf datasheet for the tube.
Different data sheet - same pinout. As I posted earlier - I think that Tim's schematic needs some proofreading.
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File Type: pdf 6C33C-B-6S33S-VExtendedDatasheetMB[1].pdf (76.5 KB, 388 views)
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Old 16th October 2010, 12:34 PM   #17
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Originally Posted by cnpope View Post
I more or less just followed Tim Mellow's design.


Chris
Chris what did you use for N1?
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Old 16th October 2010, 05:19 PM   #18
cnpope is offline cnpope  United States
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For N1, I used a 65V 0.7mA neon lamp, type A9A by Chicago Miniature. Available from Mouser for 32 cents.
Any small neon lamp is probably fine.

For N2, by the way, I'm using an A81-C90X gas discharge tube made by EPCOS. Again, available from Mouser, for about $2.00.

It looks like Tim Mellow's pin numbering scheme for the 6C33C assumes you take the anode to be pin 1, and then count round from there. The seven pins on the 6C33C are at equal angular spacing, so there is no obvious choice for which should be called pin 1. The only distinguishing feature is that the anode pin is thicker than all the others, so calling it pin 1 could be considered natural, I suppose. But anyway, since his schematic doesn't indicate the pin connections for the two heaters, you need the full data sheet to get that information, so its probably best to stick with their numbering convention.

Chris
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Old 16th October 2010, 05:38 PM   #19
cnpope is offline cnpope  United States
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By the way, I'm pretty sure that mislabelled R29 resistor is the only typo in the article (if you don't count the valid, but non-conventional, numbering of the 6C33C pins).

One other remark; I recommend using an NTC thermistor in series on the mains AC supply side of the transformers, to reduce the start-up surge. I am using a couple of hefty toroidal transformers for the output-tube HT supplies and the 6C33C heaters, and I was blowing pretty hefty fuses until I added the thermistor. I'm using a CL-40, again available from Mouser.

Chris
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Old 16th October 2010, 06:18 PM   #20
dwhitf is offline dwhitf  United States
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Quote:
Originally Posted by cnpope View Post
By the way, I'm pretty sure that mislabelled R29 resistor is the only typo in the article (if you don't count the valid, but non-conventional, numbering of the 6C33C pins).

One other remark; I recommend using an NTC thermistor in series on the mains AC supply side of the transformers, to reduce the start-up surge. I am using a couple of hefty toroidal transformers for the output-tube HT supplies and the 6C33C heaters, and I was blowing pretty hefty fuses until I added the thermistor. I'm using a CL-40, again available from Mouser.

Chris
Hey Chris!

Don't mean to beat you like a horse(with questions) but you seem "to be the man" on this project. Having broke ground on this Project and having a working amplifier.

I'm in the parts collecting phase and would love to know the "hefty toroidal transformers" you picked for this project. HOW ABOUT A PICTURE?
Thanks in advance!
dwhitf/Darrell
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