Sourcing Replacment Attenuators. (Collins Valve Gear)

studiorat

Hi Chaps,
I just noticed the forum here and I'm hoping someone might be able to make some suggestions, there seems to be a few here who know about vintage electronics. Myself and a pal are sourcing parts at the moment for a project. We are hoping to build a leveling amplifier based on the Collins 26u. It's a variable MU design where the output is linked back to control the bias voltage on the input stage.

At the minute we are trying to source parts, either the actual parts used or something very similar. The leveling part of the circuit is based on a 6386 valve, we'll be swapping that out for two valves instead. The 6386 is nearly €120 at the moment.

Anyway, I'm currently trying to find something to replace the attenuators. The only ones I can find aren't quite right and the one I can find is nearly €200 on ebay.

Here's the spec's from the original manual for the three components we need to source.
1.) 600/600 ohms impedance, 20 steps, 2 dB ea step except last. Last step infinity. CW shaft rotation; Daven Co. No. 5407

2.) 600/600 ohms impedance,, 20 steps, 2 dB ea step except taper last three, CW shaft rotation; Daven Co. Type LAQ350

3.) 7500 ohms input, 3900 ohms output, 12 steps, 2 dB ea step, cw staft rotation; Daven Co. type 350.

Has anybody ever seen these things? Is there a modern equivalent that will give us the same kind of taper etc.

Also can some on explain what the 600/600 ohms means in this regard. I can see in component 3 it probably starts at 7.5k and ends at 3.9k. Anybody know what the 600/600 means?

watty

You might actually get that 6386 a lot cheaper.

Also look at MosFets as alternatives to valves. Different bias, but otherwise very similar. Even 900V parts are only a few Euro.

600/600 is the input and output impedance.

There is free filter program at www.aade.com that can design attenuators. (each step).

It's easy and cheap to make an arbitrary impedance attenuator with rotary switch and 1% Metal Film resistors.
(or better if you want more accuracy).

You can design whatever taper of attenuation you want.

studiorat

Hi Watty,
The 6386 is very in vogue in the pro audio world at the moment. There's company called JJ making them, they are popular 'cause they were used in the Fairchild 660 limiters. Manley were using them but they don't have the stock and I suppose they have found a cheaper alternative. I'd like to have a valve in the limiting stage anyway. I'm thinking one valve wired as two triodes might match a little better than 2 separate valves?

I did notice one schematic for a similar leveling amp that used transistors to send the signal back to the input tubes, 2 x 12ax7's. I'd go for that but I reckon my buddy want to go for an all tube set-up. It means getting a big output traffo but I suppose that's part of the sound of those things too. .

Thanks for the link. That's going to be useful I hope. But I'm still a little confused about the impedance for the attenuators. How come the impedance doesn't change with each position on the 600/600 devices. Surely it's 600ohms to what ever the attenuator's highest resistance setting.
How can it be the same impedance on the input and output if it's an attenuator?

I'll check the link and come back to you...

watty

An attenuator can be 3 resistors arranged in a Pi or T circuit

 _
| |

___
 |

Considering the PI

As the middle series resister is larger, the two matching resistors either side become smaller and attenuation is greater.

If middle resistor is 0 (no attenuation) the edge resistors are infinite (not there).
If the middle is missing (open circuit) then the two edge resistors are 600 ohm each.

Thus by selection of the 3 resistors you can have 600 / 600 and any dB attenuation you want. If the input and output impedance are not the same, then there is a minimum attenuation.

The program on AADE lets specify input and output impedance (it suggests a minimum attenuation) and attenutation, It does T or pi circuits.

If you have a 3 pole 20 way switch you can wire the 20 sets of 3 resistors on it. Each position can be any attenuation you like.

watty

Re the 6386 The 5670 is similar
http://www.radiomuseum.org/tubes/tube_5670.html



I'd not use two separate triodes.

I'd use ECC82, ECC83 (12AX7), ECC85

Slightly taller and all fine for Audio. I think the the 6386 is a VHF valve so may have much shorter life than ECC types above.

The ECC88 is maybe the most similar as it also (6386) was used for VHF radio front end.

http://www.r-type.org/exhib/aai0011.htm

You might also consider ECC86/6GM8
http://www.classiccmp.org/rtellason/tubedata/6GM8.pdf

The all digit designators and 12AX7 style designators are American and ECC type designation is European

E= 6.3V filament, P = 300mA series filament TV sets.

C= triode, thus CC = two triodes
F = pentode

http://www.tubedata.info/tubnum.html
http://en.wikipedia.org/wiki/Mullard-Philips_tube_designation


Is this what you are building?
http://www.freeinfosociety.com/media.php?id=4921

http://www.collinsradio.org/archives/manuals/26U-1-_07-58_.PDF

The first Stereo amp I built about 1971 used 2x 6V6 (only).

Radiola

Hi Guys,
As Watty says 5670 is an equivalent and might have some available.

I have been a tube guy forever.

I suggest you design a pi or T network for the various levels of attenuation you need. I can possibly offer some T pads, but more importantly is the use of a good (gold plated) multi-position switch. I have a number of these if they are of interest.

studiorat

Guys! That's just pure gold. Thank you both.

I read through Watty's post on attenuators and had to mull it over for a bit, but then it came to me, I'm getting the head around it now.

So I'll try and find a decent switch like Radiola suggested.

I've been using valve stuff for ever, but working on stuff with valves in is new to me. I'm enjoying the thrill of working with a couple of hundred volts running through the gear, it's certainly going to me more careful. I'll keep looking for the rest of the parts, and I know I'm gonna be back for suggestions for a couple of decent transformers for this.

watty

You can use suitable voltage ratio mains transformers for Push Pull Audio transformers as in that design. They don't work so well sometimes in single valve driver circuits as the DC current flux can't cancel, though even there I have used 220V AC to 12V AC PSU transformer for 8 Ohm loudspeaker on a repaired radio set.

I've seen Radiola's shack. He has nearly a wall of valves, switches and transformers so use the PM to contact him. eBay can sometimes be more expensive than retailers so always shop around.

studiorat

Thanks,
I PM'd RadioLa.
The power transformer should be easy enough there's a few possible donor amps lying around the place. The Audio transformers will be the poshest we can find.

Me and my mate were discussing the 5670 the other day we might start off with that. I'd be concerned to see if it had the right characteristics in how it rolls off as it saturates or what ever you call that thing it does at high levels. More important to actually start though...

More later.