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Old 26th November 2011, 07:30 PM   #1
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Join Date: Nov 2011
Location: South Carolina
Default Christmas Build, advice needed.

Hello everyone, I'm new to this forum, but not to other audio/instrument forums.

I was wanting to build my girlfriend an amplifier this year (I made her a jewelry chest last year), and I really wanted some advice from a knowledgeable community. Preferably this would be a stereo SE KT88 amp.

I have some components already, so I was thinking this might be feasible, with a build time of approx. one month.
I'm shooting for simple here, but I have one little caveat that I feel I must have. I want a pair of 6E2/EM87 indicator tubes set up as voltage meters to give it a creative flair.

So with that said, this is my current inventory:

transformer (Power) specs
PRI- 117VAC 1.26A
Sec1- 574VCT .190ADC
SEC2- 5VAC 4A
SEC3- 3 6.3VAC 4.64A

Quantity: 2

Transformer (Output) specs
PRI 3.9k ohms CT
SEC 3 ohms, tap@1.5 ohms

Quantity: 2

Filter Choke specs
Total Direct current .190ADC
DC resistance 51ohms
Inductance (0 Current 1000~) 1Hy

quantity: 1

Interstage Driver transformer, Epoxy potted
Unknown specs, I'll have to work on this one.

A few small power transformers from old solid state amps.

Tubes:
Sovtek KT88 (2x) If these sound like crap, I'll buy better ones.
Shuguang 12ax7 (3x), I'd like to use one per channel.
NOS Raytheon 5y3GT (2x), My girlfriend likes heavier music, so I may go out and get a solid-state Rectifier to help reduce voltage sag and take away a bit of the natural delay.

I've got more sockets than I could possibly need
I've got plenty of 1950's CTS pots, including one with an on/off switch that I may use in this build.

Misc components:
Misc resistors and caps, tons of vintage cloth wire, probably just going to buy new parts anyways.


So would the specs on my power and output trannies work for a stereo SE KT88 build? If not, what kind of circuit modifications would I need to make for it to work, provided I used something like Abdellah's SE KT88 build as general guidelines?
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Old 26th November 2011, 09:30 PM   #2
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The power trafo is NOT up to the challege of energizing a PP KT88 stereoblock. PP O/P trafos can't be used in SE circuitry.

The O/P trafos might work with EL34 class O/P tubes and 4 Ω speakers, but a different (more capable) power trafo is needed. The $57 AnTek AS-4T360 seems suitable.

What's on hand does not mesh with the stated objective. Your power trafo is suitable for a PP stereoblock that uses "12" W. tubes, like the EL84 and 6V6. "12" W. tubes are usually mated to trafos with 8 KOhm primaries.
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Old 26th November 2011, 11:52 PM   #3
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I said that I was shooting for a Stereo SE KT88 amp, which I probably could have made a bit more clear.
With 1.6A draw per KT88 tube at full load, the 4A 6.3v secondary leaves plenty of room for 12ax7s, so it should run fairly well under the load of just two KT88s. At least that is to my understanding.

I have another power transformer that is a bit heftier than the one mentioned above (it has 3 extra layers of laminations), and I can't find the full specs on it, but it came from the same amp 'family'.

In both cases, these irons are very large and quite heavy, weighing approx 7.5lbs and having dimensions of 4.5" tall, 4.5" long, 3.75" wide.

If you still feel that everything I have would suck for an SE KT88 build, then I'll look into buying a quad of 6v6s/EL34s
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Old 27th November 2011, 12:49 AM   #4
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It's not the heater windings, it's the rectifier winding that's completely inadequate. Both 287-0-287 VRMS and 190 mA. of DC are the sort of numbers that are associated with "12" W. O/P types. You could do a full pentode mode "El Cheapo" variant for about 15 WPC, with your power trafo. A 5U4GB rectifier working into a 40 μF. 1st filter cap. would get the B+ rail voltage just about right. Use both 1 H. chokes (1/channel) to get excellent separation. Pile up the energy storage in the reservoir cap. positions. Good performance out of full pentode mode requires regulated g2 B. A Maida regulator set up for 315 V. gets that job done well.

Edcor offers O/P trafos with quite decent performance, at modest expense. Their $47.13 model CXPP25-6-7.6K has a 6 Ω secondary that rates to give good results with both 8 and 4 Ω speakers.
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Old 27th November 2011, 01:02 AM   #5
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That definitely sounds doable.
In the event that I cannot afford two new O/P transformers (As much as I would love them), could I re-do this to use a PPP 7355/7591XYZ configuration for ~15WPC?

7355 data shows 4k ohm load PP, and O/P shows 3.9K, and is intended for PPP use anyways.

The output transformers both came from quad 6v6 PPP mono amps from the late 1940's, and have a good heft to them.
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Old 27th November 2011, 02:33 AM   #6
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Quote:
The output transformers both came from quad 6v6 PPP mono amps from the late 1940's, and have a good heft to them.
As I previously guessed, those O/P trafos would be good with PP EL34 class tubes and 4 Ω speakers. They rate to be OK with PP "12" W. tubes too. Just use 8 Ω speakers. There is a question about sufficient primary inductance being present, for good bass behavior. However, you have plenty of magnetic headroom and the GNFB loop should correct for minor primary inductance inadequacy. Set the high pass filter at the grid of the I/P 'T7 consistent with the low freq. limit of the speakers used.

Let's total the 6.3 VAC draw for the 6 signal tubes. 2X 12AT7s draw 600 mA. 4X 6V6s draw 1.8 A. That adds up to 2.4 A. So far, the 6.4 A./6.3 VAC winding is loafing. A B- supply is still necessary. Wire the 2X 30 VAC windings of a $10 AnTek AN-0130 in series and hybrid bridge rectify with a 6BW4 and a pair of 1 A./100 PIV Schottky diodes. Put the 6BW4 on the ground (positive) side of the bridge and the Schottkys on the "hot" (negative side) of the bridge. Reduce the 5 KOhm resistor in the B- circuitry to 2.5 KOhms. Increase the 4.7 μF. lytic to 15 μF. The 6BW4 is a lot tougher than an 'AL5. Even with the 900 mA. heater draw of the 6BW4 added in, it may be necessary to put a bleeder resistor across the 6.3 VAC winding. You want the voltage measured at the tube sockets to be 6.3 VAC +/- 5%.
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Old 27th November 2011, 03:00 AM   #7
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6.3VAC secondary on the PT is 4.64A unless I missed something.

If 7591XYZs are reissue 7355s, and 7355s are compact 6l6s, then the 900ma 96.3v current draw of the 6l6s in quad (PPP) would total 3.6a, added to the 600ma of the 2x 12ax7s would result in 4.2a of current draw on a 4.64a supply. At least I think that's the case.

Now if the rectifier winding is completely useless, then the AnTek AN-0130 could presumably supply enough B+ voltage for a quad of 7591XYZs/7355s/6l6s, (which should have a max of 500v) and the 240-260v needed for the 12ax7s?

The 6BW4 has a 6.3v heater, with a 900ma draw, right? That would exceed my PT's ratings with my proposed signal tubes, so would it be possible to find a rectification circuit that used the 5v 4a secondary winding instead? If a 5v circuit isn't an option, would it be possible to just add a small 6.3v filament transformer?

The 2-way speaker pair I have designed and made calculations for is based around 8 ohm drivers and tweeters, the specs of which have already been used to calculate crossover components and optimal enclosure design, so I'm not in the eagerest of moods to go redesign that, but if necessary I will.


Thanks in advance for your help and dedication, I'm sure I would have screwed something up quite irreparably by now if I hand't stopped to ask.

My reason for wanting to use 7591XYZs is that they are aesthetically satisfying, big, very cheap, and I really love them in my Stromberg Carlson APH1030 amp.

Last edited by Tres Maddox; 27th November 2011 at 03:14 AM.
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Old 27th November 2011, 03:04 AM   #8
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And I'm presuming that in that case I would have to go and buy new O/P transformers if I were to go with that tube arrangement?

Which might be financially acceptable given that I won't have to buy a new quad of tubes, only a very cheap pair, as I already have two in my other amp I can borrow for the gift.
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Old 27th November 2011, 04:21 AM   #9
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IIRC, the 7591XYZ is a rebased 5881. That's a "19" W. tube that needs over 400 V of B+. The 267-0-277 VAC rectifier winding of your power trafo, along with its 194 mA. DC current rating, forces the use of "12" W. tubes. Live with 15 WPC or buy all sorts of new "iron". 6L6 family tubes work with 6.6 KOhm primary O/P trafos, not the stuff you already have. By using a PP pair of 6V6s in each channel, along with 8 Ω speakers, you limit your magnetics purchases to the $10 AnTek torroid. Magnetics, not tubes, are what jacks the price of "hollow state" amp building up.

Current production ElectroHarmononix (EH) 6V6s are quite decent. For a little more money, you can buy another Saratov made product, the EXCELLENT "reissue" TungSol 6V6. Poindexter, no fool he, prefers the TS "reissue" to NOS.

BTW, we have not talked speakers and that's IMPORTANT. Does your lady already have some speakers? If so, what are they?
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Old 27th November 2011, 04:44 AM   #10
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Alrighty, sad thing is that I just shipped off a matched Quad of EH6v6GTs

She has no speakers, and that's what I was working on building. I've already completely design the enclosures and cross-overs specc'd to the drivers I selected, but the speakers were extremely inexpensive, and now that I'm saving a bit here, I think I might be able to splurge a bit more there.

So here's the list and prices I'm compiling:
NOS Matched Quad of Russian Black glass 6v6GTs ; $20
2x EM87 tubes : $10
2x 12ax7 tubes: Have
Antek Iron: $10
Schottky diodes: $2
6BW4 Tube: $8 for 2
Choke identical to the one I already have: $?
I still need individual components, but I guess that's awaiting a schematic

I worked out a speaker BOM and cost list, and arrived at roughly $35 for the pair, since I already have enclosure materials, but I have a strong feeling I should spend more.
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