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Old 28th August 2008, 08:04 PM   #11
phil_2 is offline phil_2  United Kingdom
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Thanks again Eli for walking me through this, I really appreciate your time and effort and knowlege. This amp must be nearly there now, I'm really pleased with how its progressing. I'm learning a lot too, I read up on Miller capacitance, interesting stuff, the big revelation for me was, while reading about things you've mentioned, finding out about the different modes you can build your amplifier to work in (triode, pentode, UL, SE etc)and the pros/cons of each, this is a big leap forward for me and my understanding.

Right, back to business, I followed your instructions and modified my schematic during my lunch break today, here's what it looks like now:

Click the image to open in full size.

is this ok so far? I think the only change you mentioned that I didn't put in was "That cathode bias part has to be bypassed by a large electrolytic capacitor." I just wanted to check, do you mean in parallel with R5? what kinda value is 'large'?

now the replacement driver tube, I've had a look and there are lots to choose from, different brands, different measured values for tested tubes, even different sizes! I was about to get these but I thought no harm in checking first, are these up to the job?

http://cgi.ebay.co.uk/ws/eBayISAPI.d...MEWA:IT&ih=001

thanks again, Phil.
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Old 29th August 2008, 01:27 AM   #12
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Phil,

If you are going to buy new driver tubes in the US, I suggest you purchase from Jim McShane, as he is a dealer with a sterling reputation. For not much extra, you can have black plate RCA 12AV7s, which will be quite yummy in combination with the Mullard made EL42s. Look closer to home for Dutch Philips E180CC, which are fine, but importing RCA from the US might cost less. Perhaps you can find CV10175s "locally". That is a British designation and could be the best deal.

The schematic is shaping up nicely.

Yes, R5 (whatever its new value turns out to be) gets bypassed. The "pole" frequency for that combo must be no greater than 5 Hz.

When the new value for R7 is determined, use an inductive wirewound part. Connect a 120 pF. mica or NPO ceramic cap. from the "hot" speaker terminal to ground. Between HF peaking (due to the inductance of R7) and the shorting of the NFB loop out above 80 KHz. (by the 120 pF. cap.) the NFB loop gets reasonably well phase compensated.

R12 and R13 should be low noise 470 KOhm metal film parts. C5 and C6 should be 100 nF./400 WVDC film and foil. 716P series "orange drops" are up to snuff for C5/C6.

Notice that R15 is not replicated at the control grid of V2. It should be. Use 10 KOhm Carbon comp. parts for R15 and R15 prime. Carbon comp. resistors are used as control grid stoppers because they are both non-metallic and non-inductive. "Horses for courses."

Good technique does not use the chassis for signal grounding purposes. Either the bus or the "star" method should be employed. The signal ground gets connected to the chassis at single point.

Don't forget to install a proper, 3 wire, safety grounded, power cord.
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Old 30th August 2008, 03:22 PM   #13
phil_2 is offline phil_2  United Kingdom
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Ok, I've Added C2 across R5, C8 across the output and cleaned up the schematic a bit:

Click the image to open in full size.

I've also drawn up a parts list to keep track of the recommended component types etc:

Click the image to open in full size.

A couple of questions, for the resistors are 1W rating ok? and the original smoothing caps C4 and C7, can I increase them to say both 47uF? I figure bigger is better with smoothing caps and I have some 47uF 450Vs lying around.

I intend to wire it 'star' ground and it will have the correct 3 core cable and fuse, no problem.

thanks, phil.
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Old 30th August 2008, 06:23 PM   #14
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Phil,

The schematic has shaped up very nicely. However, C8 should be 1200 pF., not 120 pF. That's my error, sorry. The 1100 Ohm sum of R17 and R6 combine with C8 to form a low pass filter. After allowing for the inevitable stray wiring capacitance, that filter's "corner" freq. will still be above 80 KHz.

Unless otherwise indicated, all resistors should be 1/2 W. rated. Carbon resistors come in 2 "flavors": composition and film. Carbon film parts are slightly inductive, which makes them unsuitable for grid stopper service. OTOH, Carbon film parts will be dandy in the 100 Ohm triode "strap" positions. Metal film resistors are inherently low noise and normally come with a 1% tolerance.

Vacuum rectifiers, like the EZ41, are intolerant of large valued cap. I/P filters. They will destructively arc over at turn on. The EZ41 Data Sheet indicates which situations are safe. The 25 μF. OEM value for R7 seems well thought out. The closest modern standard value is 22 μF. and that's what you should (IMO) use. The way to improve smoothing and increase energy storage, in the PSU, is to insert a LC section of 1 to 2 H. and 1 of your 47 μF. parts after the initial filter capacitor. R16 and C4 decouple the small signal circuitry from the power O/P stuff. 15 μF. should be quite sufficient for C4.
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Old 31st August 2008, 08:14 AM   #15
phil_2 is offline phil_2  United Kingdom
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Thanks Eli, updates have been noted and will be included in the next revision also an email has been sent to Jim McShane, thanks for the recommendation.
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Old 31st August 2008, 02:11 PM   #16
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Phil,

In my previous post, I said "25 μF. OEM value for R7 seems well thought out". R7 should read C7.

In the latest schematic iteration posted, you still show the EZ41's heater as being connected to the chassis. Along with the signal tube heaters, the rectifier's heater should be "lifted" and hooked up via a tightly twisted pair.

Should it prove necessary to ground the heater supply, a center tap will be fabricated from a pair of resistors or a trim pot. and the connection to ground made by a 'lytic. When this type of circuitry is employed, biasing the heater supply up to approx. 90 VDC, with a resistive voltage divider across the B+ rail, is "routine".
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Old 1st September 2008, 07:12 AM   #17
phil_2 is offline phil_2  United Kingdom
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Oops yes I missed the rectifier tube heater, again it will be included on the next revision.

I've come across a bit of a problem, well a few actually... I Went through the amps yesterday, testing the output transformers to make sure they matched...

The first problem is that the output transforms are marked 1.5 Ohm on the secondaries, they must have driven two 3 Ohms speakers in parallel (interestingly they actually both measure 0.5 Ohms across the secondary terminals?) so its going to be awkward to find such low impedance speakers, I would probably have to make my own.

and the second problem is that one of the OTs is open circuit on one half of the primary. Strangely I still got 'music' when I last tested the amp a few weeks ago, so either I was listening to one half of the output or it has broken since testing...?

oh and there is a THIRD problem (which I already knew) with the OT's but thatís just cosmetic, one (the working one) has been dipped in pitch which is flaking off and looks very ugly, I was wondering what to do about this, Its not gonna look good on top of the chassis.

Either way I figure this is quite a big deal. The output transformers are one of the main reasons to start with an old chassis instead of from scratch...

So my question is how cheaply can I replace these OT's? they are 16k across the input and I would like ideally 8 Ohms across the output. Is something like this available? I know there is a lot of variation in quality in OT's but at the same time this was supposed to be a low budget 'project'.

another concern (which might be undue but I'd better mention it) is that the output tubes of both amps are almost completely silver, is this normal or a sign that the tubes are on their way out?
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Old 1st September 2008, 02:41 PM   #18
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Phil,

O/P "iron" is a BIG deal. It's heavy. So, long distance shipping of otherwise modestly priced items can turn out to be no bargain, at all. "Local" suppliers frequently wind up costing you less, in toto.

The UK trafo winders I'm familiar with are Sowter and Stevens & Billington. Sowter's model U063 has the correct impedances, but its power handling capability is questionable. Brian Sowter is a very helpful fellow. Send him an EMail explaining your situation. Something similar to the U063 on the next larger size laminations is a distinct possibility. While you are at it, consider ultralinear taps.

I'd like to think that the circuitry under discussion is highly competent. So, investing in decent O/P trafos will pay off.

BTW, that 1.5 Ohm impedance you came up with makes sense. Paper cone speaker drivers with a 3.2 Ohm impedance were ubiquitous in 1950s consumer electronics. The OEM diagram shows a pair of drivers connected in parallel.
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Old 1st September 2008, 06:59 PM   #19
phil_2 is offline phil_2  United Kingdom
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Thanks Eli, I will look into these brands, unfortunately first impressions are that the Sowter transformers are a lot more expensive than I was hoping, even the 4W ones are over £90 each!

Hammond and Edcor OTs seem more in my price range, but I seem very restricted in my choice of OTs with an input impedance of 15-16kOhm. Can lower impedance OTs be used?
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Old 1st September 2008, 09:12 PM   #20
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Phil,

The EL42s "handcuff" you. You need to get close to the OEM impedance to use them.

About the only thing "Cheap Charlie" that comes at all close is the Hammond 125E. The spec's of that part are very much LOFI. NFB is a tool, not a magic wand.
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