KT88 / 6550 Bias Questions

[Ray Ivers]

Bob,

1400 VCT at about 650mA for a choke input filter...

After seeing high power choke-input supplies in action in two old Vox amps (4120 and 7120)... well, I just don't think I'd ever risk trying one; even with a shielded sub-enclosure, that choke threw hum everywhere. IMO the pi filter (same type - but not values - used in the Sunn 2000S, etc.) is equal in regulation - and superior in every other way except cost/size - to the choke-input when implemented with large choke/cap values, but to each his own.

I'm sure Heyboer would have no problem winding you a suitable PT - even w/separate secondaries for each channel - and OT's too if needed.

You might also check with Bob Risson - here's an eBay auction link, and at the bottom of the page he has NOS Fender 300PS OT's for sale (I bought one of these and a 400PS PT from him last year):

http://cgi.ebay.com/6550-Output-100-...QQcmdZViewItem

Ray

[Steve Conner]

Yay, I bought one of those 300PS OTs too.

[bob p]

did you use it in an MI application or a Hifi application? i'm too timid to assume that Fender's MI OTs would work well in a HiFi stereo amp.

[Steve Conner]

I've not used it in anything yet, been too busy with work I intend to use it in a huge bass amp that can also be used for guitar when I'm in a mean mood and have six 4x12"s handy

If you need to know anything more about it, I can measure the usual transformer parameters for you. Perhaps the biggest PITA is that the secondary can only be wired for either 2 or 8 ohms (two identical windings that can be series'd or paralleled)

[bob p]

thanks Steve -- knowing the specs would be nice. if you have the time to add something like that to your "to do" list, i'd certainly appreciate your effort.

i am definitely on the lookout for iron that would be suitable for a hifi stereo amp, so specs are more important in this case than they would be if i were building the BFGA.

as it turns out, the friend that i'll be doing the build for now has a piar of Linn Kann speakers to use with the amp -- and as Murphy's Law would have it, they have a nominal impedance of 4 ohms. (fwiw, i have already warned him about the potential need to buy another pair of speakers.)

[Steve Conner]

Hi Bob,

I tried some rough measurements on it with a signal generator and scope. I've not characterised an OT before, and I lost my calculator and worked it all out on paper, so I could have got something wrong here.

Size: Big. Black.
Weight: 7kg

Primary DCR: 10+9 ohms

Turns ratio: approx 40CT:[1+1] (two identical secondaries)
Impedance ratio: approx 3200:8 (or 3200:2 with secondaries in parallel)

Magnetizing inductance: approx 400H referred to primary
Leakage inductance: approx 400uH referred to primary

Saturation: About 2 volt-seconds. This implies that it will start saturating when delivering 300W into 8 ohms at 25Hz. I measured this by driving it with a 10Hz square wave from a 50 ohm source and observing how much it could pass before seriously drooping. The saturation looked fairly soft, so it could probably be pushed a good deal further.

My overall assessment is that this is a pretty serious output transformer, and an amp built around it could probably topple small buildings with a 5 string bass. I just got a PT from an old ham radio boat anchor to go with it today, that gives a choice of 320, 600 or 850V B+.

[bob p]

thanks for your help, Steve. I've never characterized an OT either, though I have been reading about leakage inductance, phase shifts, negative feedback, oscillation, etc. I really appreciate your efforts.

[bob p]

Ray, would you happen to know the secondary voltage rating of the mains transformer on your Mac 75? I'm thinking about PT options for a Pi filter based amp, and I'm curious about the values in the MC75 at the PT secondary, and at the output of the votlage doubler (at the input to the choke). I'm thinking that if I spec a large PT, getting into the range of the Mac voltages without a voltage doubler would be a good option.

MC75 schematic attached for reference.

[Ray Ivers]

Bob,

For 460V out, the MC-75 HV PT secondary should be somewhere around 180VAC.

Ray

[bob p]

Thanks! Is that a modeled number or a measured number? I was getting about 190V with my PSUD2 model. (Its always good to have your numbers verified.)

Back to the original question about choke input vs. cap input, instead of relying on "napkin math," I've finally gotten around to modeling some numbers. Surprisingly, after crunching the numbers for some off-the-shelf Hammond iron, the results come out pretty much the same -- regardless of whether I use a 625-0-625 / bridge / choke input configuration, or a 400-0-400 / bridge / Pi filter. In the first case it looks like I get 564V at idle (539 at full load), and in the second case it looks like I get 529V at idle (493V at full load).

The difference in power output isn't that significant between the two cases: 90-100W vs. 85-95W. It looks like it could be feasible to go either way with the stock Hammond iron.

Could somebody check my math?

One of the other things I'd like to ask about is the bias voltage source. The 700 series Hammonds don't have them, so it would probably be necessary to take the bias voltage off of the secondary using a voltage divider or a capacitive divider.

At least the Hammond 300 series units (like the 378cx) have the 50V bias tap. I'm thinking that 50V is pretty small, and that I'd have to use a voltage doubler to get into the range of the -60 to -75 volts of grid bias needed for this application.

If anyone has recommendations/comments along these lines, specifically about potential pitfalls with either of these biasing methods, I'd love to hear from you.

[bob p]

what are your preferences for new production tubes?

Well. I recently did some very basic power/waveform testing with the EH, SED, Valve Arts, and one other KT88 I can't recall - plus the KT90EH - in a Marshall Major UL circuit, under "semi-torture-test" conditions (two tubes, 8 ohm load on 8 ohm tap). IIRC the KT90 put out the most power with the best-looking waveform, followed by the rest in pretty much a 3-way tie. I've been using SED (Svetlanas when I bought them) KT88's in a '73 SVT for years with no problems and great sound. The rest of my KT88 amps all have either GECs or Nationals in them.

resurrecting an old thread, i went to one of the obscene HiFi shops in Chicago last weekend and had a chance to listen to the Audio Research reference series amps while sampling an array of very fine speakers. i was a bit surprised to learn that the AR amps are populated with the EH 6550C. needless to say, the amp sounded great.

it didn't surprise me that a high end US company would choose 6550C for their output section. what did surprise me, though, was that they were using EH branded 6550C in their amps. i would have expected EH to have been producing a KT88.

does anyone know if there's any real difference between the 6550C EH and the KT88 EH? Or between the SED 6550 and KT88, for that matter? Is there any real difference in these tubes, or are we just looking at a bottle/envelope and label change?

[ecc83]

KT88 bias

I would like to throw in a few points re the good old '88.

Since they are expensive and can pull large anode currents the bias should be well balanced (best LF performance).

Were I to go back to valve amp construction I would design the fixed bias feed such that it fails "high" if the pot wiper goes o/c.

I would also fit cathode R's of about the right value and then bridge them with 10.0Ohm fusibles which would burn out in the event of a bias failiure and save the bottles. Nice blue LEDs on the front to tell you!

Dave.