Mullard schematic?

[petera77]

I'm looking for a mullard circuit schematic, preferably using a pair of 6550 for about 60W a channel.

TIA

[richwalters]

Mullard UK (as far as I know) didn't venture into 6550 US terroritory. You'd be hard pressed finding any design.

One solution is to examine a KT88 design and fit 6550 tubes instead, observing lower B+ in UL mode and slight differences in fixed bias volts....I wouldn't go so far to say these tubes can be "swapped" in sockets, but with so many power tubes, working conditions can be trimmed to give same output power..

In 1960's Mullard did a 50W fixed bias design using EL34's which was little heard of, but targetted for MI work.

richj

[pchw]

This is the Eico HF-50/60 schematic. It is a variation of Mullard 5-20 using 2 EL-34, but it can be used for KT-88 or 6550 with some mods. 50 watts for the HF-50, 60 watts for the HF-60. The 10 watts difference is due to different OT's - Chicago iron and Acrosound 330.

[tubewade]

I can't report any experience with this circuit but here is a Mullard implementation using 6550s that you might enjoy looking at.

Wade

[tubewade]

... and another using KT88s. Could be configured for 6550s.

[kevinkr]

Were I to use that Japanese implementation I would be concerned that they used a half wave rectifier for the B+, I'd go with a bridge. (Post 5)

Note it also uses the much maligned 12AU7A in the driver position. You could sub this directly with a 12BH7A which is slightly more linear.

[Nigel Goodwin]

Originally posted by tubewade
I can't report any experience with this circuit but here is a Mullard implementation using 6550s that you might enjoy looking at.

That looks almost identical to the "Mullard 5 Valve 10-Watt High Quality Amplifier - Distributed Load Version" in a small 1958 book I have called "Quality Amplifiers for AC Mains from Data Publications". It uses EL84's as outputs as usual.

[tubewade]

Originally posted by kevinkr
Were I to use that Japanese implementation I would be concerned that they used a half wave rectifier for the B+, I'd go with a bridge. (Post 5)

Note it also uses the much maligned 12AU7A in the driver position. You could sub this directly with a 12BH7A which is slightly more linear.

I agree on both counts. Instead of 12AU7 I'd be more inclined to use 6SN7 or 6CG7/6FQ7. 12BH7 would be fine also. The circuit layout is what I was hoping to provide.

Regards, Wade

[JoshK]

Tha Japanese 6550 version looks like an interesting amp to try. I'd probably use the EF37A's I have for the first tube and keeping with an all octal theme use either some 6SN7's or maybe the EH 6H30-octal tube for the driver.

But I got too many amps to build now...