Potential project for some inherited transformers

[tikiroo]

Fantastic response Eli, you've answered all my questions before
I asked them! Plenty of information to get me started. I'll have a go drawing some load lines using the triode curves for the JJ 7591

Originally posted by Eli Duttman

The 3rd filament winding will be voltage multiplied along the lines of the 2nd schematic shown here. All 6 stages form the B- supply needed by the splitter/driver circuitry.

I had thought of doing that, but couldn't work out the required voltage for B- from the circuit diagram, thanks for the tip. What value capacitors should be used in the voltage multiplier?

What sort of signal source(s) are planned for use with the ECG monoblocks?

Just a CD player, I don't own any vinyl. So plenty of signal level.

Has consideration been given to volume control, channel balance, and source selection?

Not yet. Probably just a basic volume pot.

Do you have something like a 6X4, 6X5, or 7Y4 in your parts pile?

My "parts bin" (at least as far as tube gear goes) consists of the transformers, a few tubes salvaged from the original amplifiers (Mullard and Miniwatt EF86, ECC82/83, EL84, EZ81) and....that's it! I'm sure I'll be able to track down a suitable rectifier valve.

A week ago my knowledge of vacuum tube electronics was next to nothing, isn't the internet amazing!

[Miles Prower]

If those 8K Partridges are push-pull OPTs, that would work just great with 6CK4 finals. I was considering this myself, but all the junk box pickin's suggested doing a 6BQ6GTB design instead, since I had the OPT and power xfmr for that.

You get the advantages of avoiding any sort of screen supply, 6CK4s are real triodes, so you don't have to triode pentodes. They seem to work real good here. Since this was designed for TV vertical deflection duty, they should also be pretty tough.

6CK4 PP Loadline

[Eli Duttman]

Steve,

The triode curves on the JJ data sheet provide useful info. Notice that when the plate to cathode differential is 400 V. and I_B is 30 mA., the negative grid bias is between 20 and 25 V. A drop of 6 V. will occur thru the shared 100 Ohm bias resistor at that "idle" current in each of the PP pair. Somewhat less that 20 V. has to come from the C- supply. Approx. 7.5 V. can be expected from each Schottky diode stage of the voltage multiplier. So for bias, 4 stages will prove adequate. 270 KOhm grid leak resistors and a 100 KOhm trim pot. seem to be satisfactory. The wiper of the trim pot. connects to the "bottom" of both grid leak resistors. The ends of the trim pot. connect between C- and ground.

An additional 3 Schotkky diode multiplier stages will be quite sufficient to drive the B- rail. At the low current draw in the negative rails, 330 μF. 'lytics will crush the ripple fundamental, in spite of the 1/2 wave situation. However, large filter caps. cause ripple overtones to be present. Those overtones will be killed by inserting RF chokes (RFCs) in the lines leading away from the voltage multiplier. A final reservoir cap. will complete each of the negative rails.

The EZ81/6CA4 rectifiers you inherited are "boffo" for the B- delay role. Connect the RFC that leads from the 7th multiplier stage to the vacuum rectifier's cathode. Connect the rectifier's anodes in parallel to a 47 μF. reservoir 'lytic.

[tikiroo]

Miles,

That 6CK4 does seem to have a good fit, but they might be a bit hard to find in this neck of the woods. Also the drive voltage swing required is a lot higher than for the 7591's so I would have to re-think the driver stage which is probably beyond me at this stage!

Eli,

Thanks for the further information, I think I have everything I need now. I'll draw up a schematic and start tracking down the required bits and pieces.

I've had a go at the load lines for the triode connected 7591. At a Va of 370, 8k a-a load and idle current of 45mA per tube, max power is 9W (8W class A). The 6k taps look like they will be too heavy a load, although as mentioned previously there is the possibility of using them as "ultralinear" taps to squeeze a bit more power out of the tubes.

[Eli Duttman]

Steve,

You may have observed that things would improve, if the B+ rail voltage were to be higher. If you are willing to invest (not too much ) in some additional "iron", you can get more B+ Volts from the trafos already in your possession.

Use the "identical" pair of 6.3 VAC filament windings to extend the rectifier winding ends. Use the remaining filament winding in the series boost connection, on the primary side. Such an arrangement will get the B+ rail above 400 V.

[tikiroo]

Thanks for the suggestion. Alternatively, the power transformer primaries have 10-0-200-220-240 taps. If I use our nominal 240V on the 0-220 volt taps, all secondary voltages would be boosted by 9%, getting B+ over 400V. Although all the heaters would all be running at 6.9V which might be a problem.

Va of 400V would give 11W power (6W class A), probably not enough to notice the difference, and about the same as I would achieve if I used the 6k (85%) taps for ultra-linear operation.

I'll keep to the standard arrangement for now, but these are options for experimentation later on.

[gingertube]

tikiroo,
I'm in Adelaide but was a farm kid from Kybybolite and went to Naracoorte High School which is only 50 klicks up the road from "the Mount" where you are.

If you need advice, supplier recommendations etc. or if you ever get to the "big smoke" and want to hear a Baby Huey give me a buzz.

I'm actually running some $3500 Audio Space 300B Push Pull Monoblocks at the moment. The owner has asked me to see if I can't improve them a bit - which they need. The Baby Huey leaves these supposedly Hi End things for dead. You don't need to spend a fortune to get great sound.

My "Baby Huey", Eli's "El Cheapo", Poindexters "Music Machine", SY's "Red Light District" (and possibly some others) are all aimed at this end of the DIY market. Getting great sound without spending huge bucks.

Home most evenings and weekends. 08-8269-3539.

Cheers,
Ian (Gingertube).

[tikiroo]

Hi Gingertube,

I really appreciate the offer of advice (and the phone number!). I'll be sure to give you a call if I get stuck.

I'm just getting together a list of all the bits and pieces I need. I should be able to scrounge high voltage power supply and other capacitors from work out of scrapped industrial DC drives - as you have guessed I'm trying to do this on the cheap!

Are there any local suppliers of valves etc you would recommend, or is overseas (eg Antique Electronic Supply) the best value?

Steve

[gingertube]

Steve,

Australian Suppliers rave:
Do a web search for Bob's Tube Audio.
Bob Rundle is an ex Electronic Engineer and Tech. College Instructor who I first met nearly 35 years ago when he was running a company which designed and built ECG machines (I did my electronics training in medical electronics at the Royal Adelaide Hospital). He is now semi retired and runs an on-line tube store. Trades on e-bay as "qch" and opens his store (Jaybird Electronics) on Port Road, Friday and Saturday afternoons. There is an email contact on his e-bay listings. The store openings are as much a social event (gab fest for tube nutters) as a real commercial thing. He does most buisiness on-line. You can contact him on 08-8447 7411. He is a Hammond agent for trannies and chassis, and a JJ Electronics agent for tubes. Also has a range of capacitors, tube sockets, parts kits etc he gets in from Taiwan and a constantly changing range of NOS and used tubes which all go through his AVO tube tester before being offered for sale. Often has stuff like tube testers, vintage radios etc for sale on commision.

There is also Evatco (also on the web) who are now in Brisbane. Evatco are a fair bit more expensive but have a larger range and keep most of it in stock.

I buy most of my stuff from Bob but have also used Evatco quite a bit. Overseas suppliers I've used are PartsConnexion and Antique Electronic Supplies, had good service and quick delivery from both.

For example, I recently needed 2 matched quads of EH 300B's. It was cheaper for me to get them from PartsConnexion in Canada (who had them on special) than to get them from Evatco who are the local ElecroHarmonix agents. On the other hand when I needed 2 matched quads of Gold Lion Re-Issue KT88 it was cheaper to get them from Evatco (by the time I factored in delivery and GST).

Cheers,
Ian