New to me paraglow monoblocks

[superchunk]

Hi all, sorry if this is the wrong space for this but I couldn't find a better one.

I just picked up some old paraglow monoblocks, with the cobalt transformers. I'll be moving them to a new chassis and I was wondering if there was any circuit upgrades I should be making, perhaps power supply? Is there a repository for this kind of info (revisions etc...)?

Happy to hear any and all thoughts on these. I'm a novice with a little bit of experience with kits etc ..but looking to learn learn learn.

 

[Paul Joppa]

That was from the very early days of Bottlehead; in fact I'll have to check my files to see if it was before the name Bottlehead was adopted! There doesn't seem to be a specific sub-forum under the "Legacy Products" forum - we'll have to make one. If I recall correctly, that was the first time we used parallel feed output transformers. It is a parafeed version of the Afterglow direct coupled 2A3 amp. I see it has been modified - the big capacitor glued to the side panel was not stock. A few more photos, of higher resolution, would help us identify any other mods and version - I think there were two different power transformers used, and the driver was probably a 5965 or a Russian 6N1P.

It's a rare beast indeed, and the cobalt transformers rarer still.

I'll dig up my old notes, but in the meantime just note there are probably a few changes that should be made for safety and longevity.

 

[Jameson]

Created a new sub forum and moved the topic there.

 

[superchunk]

That was from the very early days of Bottlehead; in fact I'll have to check my files to see if it was before the name Bottlehead was adopted! There doesn't seem to be a specific sub-forum under the "Legacy Products" forum - we'll have to make one. If I recall correctly, that was the first time we used parallel feed output transformers. It is a parafeed version of the Afterglow direct coupled 2A3 amp. I see it has been modified - the big capacitor glued to the side panel was not stock. A few more photos, of higher resolution, would help us identify any other mods and version - I think there were two different power transformers used, and the driver was probably a 5965 or a Russian 6N1P.

It's a rare beast indeed, and the cobalt transformers rarer still.

I'll dig up my old notes, but in the meantime just note there are probably a few changes that should be made for safety and longevity.

I'll grab a bunch of pics today. This one has the exo-36 and is the one Bill Gaw built for his article long ago.

Yeah those caps are probably 60+ years old. They won't be staying.

Because of the quality of the transformers it'll be a cost is no object on parts as I rebuild it.

 

[superchunk]

 

[Paul Joppa]

I found three draft notes, documentation I've been slowly working on. I believe PB has done some refurbishing as custom work as well. Note, there was a Paraglow II incorporating John Tucker's early implementation of shunt regulation for the driver. It was a non-Bottlehead modification kit.

 

[superchunk]

Ah, super interesting. Awesome to have those notes. Incidentally I had a John Tucker phono stage for a while, sounded very nice. Should have kept it probably.

 

[superchunk]

Oh, btw, I have the original booklet and thought I would mention that if none of yours survived the 30 years, and it has any sentimental value to you I can scan it for my own needs and send it to you (anyone at the bottlehead team).

Just a thought since it's possible few are still out there.

 

[superchunk]

 

[superchunk]

Ok got these hooked up to try out today, and holy crap. These are some of the nicest amps I've ever heard. Closest comparison I could make would be to a triode labs parallel 2a3 I had, which was excellent, but the clarity and inner detail is even better on these. I'm excited.

I'm compiling a list of various upgrades I have seen, hoping some experts will be able to take a look at that when I'm done.

 

[Doc B]

That gave me a flashback. That run of transformers was unique. In or around 2001 Mike LaFevre asked me if I wanted to do a special run of cobalt transformers, as he had acquired a small ($$$) quantity of cobalt steel laminations. I don't recall production numbers but there were EXO-35s, 36s, 45s and 46s IIRC. Probably not more a few dozen total. One thing led to another and I ended up spending hours upon hours in the kitchen of the old farmhouse we had leased for that year, polishing the brass channel frames and flocking the bobbins. Yeah, only did that once...

The cobalt lams were interesting stuff. Whereas the permalloy transformers we had been selling had a great sense of delicacy but weren't really great at big transients, the cobalt could be really punchy with the tradeoff being a little less fine resolution. Not long after this Mike started mixing permalloy and M6 lams in an attempt to get the best ratio of fine resolution and dynamic capability.

I'm remembering that Mike felt you could push the cobalt quite a bit harder than M6. You might consider a circuit that can put out a little more power than the standard Paraglow. It was around 2.5W, limited by the direct coupling of the stages. Seem to recall Mike felt you could put more like 4W through them. PJ should probably weight in on this.

 

[Paul Joppa]

Yes, some really great sounding amps came out of the late 1990's experimentation. One of them is the cobalt core, which provides more headroom for clean transients.

Looking forward to seeing your list!

 

[Doc B]

Also, the 450V rated filter caps were really at their voltage limit. When we would get those in for refurb I would replace them with a stack of a couple of 450V rated caps in series, with 100K ohm equalizing resistors across each one to handle the startup voltage spike. Bear in mind that for two caps in series the capacitance goes down to c1c2/c1+c2 so you would need twice the capacitance rating for the new caps to retain the original capacitance spec.

And I recall that the cathode resistors on the 2A3s were marginal in terms of power rating too. I seem to recall around 30W minimum is what they need to be.

 

[2wo]

Wow, the cobalt transformers were really rare. It would be worth optimizing the entire amp around them.

For the filter caps consider some of the DC link film caps.

 

[superchunk]

I have to say, to my tastes and I'm my application this amplifier is an absolute triumph. I've had a lot of amps through my system in the big $ big name range (dartzeel type stuff) and this one easily just knocked my reference down to midbass duties. I was on the verge of tears last night listening to music.



Again, I'm a total novice in electronics so I'll be going slow and easy. And I like it so much I maybe don't want to make major changes.



My list so far:



1. Replace 4uf motor run with mundorf 6.8



2. Fix up the soldered fuse



3. Replace RCA with kle - and wire neotech



4. Replace the 100uf caps with motor run or 2x 200uf + 350v+ and resistor to equalize voltage (wattage for resistor?)



5. Clean up the IEC wiring - replace with duelund 600v



6. Replace the 3k 12w resistor with 3k 50w rieden from connex



7. Wire mute switch to also turn off



8. Use rod Coleman constant current board?



9. 0.15 duelund bypass caps on all capacitors



10. Change driver tubes to 6sn7?

 

[Paul Birkeland]

6. Replace the 3k 12w resistor with 3k 50w rieden from connex

10. Change driver tubes to 6sn7?

You'd want to use a 100-200W aluminum housed resistor if that's the way you want to go. Those resistors need a big aluminum heatsink to work properly (bigger than your top plate), so they have to be seriously derated otherwise if you don't have that much metal available. I'd recommend going to Mouser and looking for tubular wirewound resistors instead.

The 6SN7 will drop the gain of the amp a fair amount, and you'd have to do some analysis to determine an appropriate operating point that won't limit the output power of the amplifier.

 

[superchunk]

You'd want to use a 100-200W aluminum housed resistor if that's the way you want to go. Those resistors need a big aluminum heatsink to work properly (bigger than your top plate), so they have to be seriously derated otherwise if you don't have that much metal available. I'd recommend going to Mouser and looking for tubular wirewound resistors instead.

The 6SN7 will drop the gain of the amp a fair amount, and you'd have to do some analysis to determine an appropriate operating point that won't limit the output power of the amplifier.

ok thank you, that's coming off the list, was a passing fancy. I'll be starting with the easy parts updates and upgrades, and reading Morgan Smith and Reiner van Lunde (sp?) over the winter.

 

[Paul Joppa]

I would LOVE to get a copy of the manual! I have only pages 8 and 9 at present. Thanks for the offer.

The situation with the driver is confusing The circuit on page 9 shows a 10mA current source as used in the Afterglow driving paralled triodes of the 5965 driver with a 182 ohm cathode resistor. But the Paraglow circuit is a single triode drawing 5mA with a 274 ohm cathode resistor. I know that at some point we decided that the single triode sounded better, so that's probably what you have and the C4S diagram is wrong.

To further complicate matters, you have the PGP8.1 power transformer which puts out more voltage - at one time I calculated 53 volts more, but never actually measured it. This causes the 2A3 plate dissipation to exceed the spec, shortening its life.

You can "spend" that extra voltage on either the 5965 or the 2A3, or both. Many modern-construction 2A3s can handle greater dissipation, and will deliver more power that way. But I'd recommend giving at least some of it to the 5965 for greater driver headroom. In any case, the cathode resistors of one or both tubes would need to be changed, and possibly the current source setpoint resistor too.

Can you confirm that the 5965 is using only one of the two triodes and has a 274-ohm cathode resistor? Can you measure the plate and cathode voltages of both tubes?

 

[Doc B]

That's not a mute switch. It's to delay the B+ hitting the plates before the filaments have a chance to heat up. This is for prolonging tube life and it will also happen to keep the startup surge from slightly magnetizing the output transformer core. That seems to be a possible cause for the slightly strained sound of a tube amp before it warms up, i.e., plays some music and thus the output transformer core gets demagnetized by the music signal.

I'm thinking I gave a hardcopy of the Paraglow manual to Jameson or Eric, in a pile with some other legacy manuals.

Oh and I wouldn't mess with the 5965. It's a great tube, pretty much the equal of the 6SN7 in terms of linearity. And it has higher gain which is a plus for a two stage amp with a low mu output tube.

 

[superchunk]

Paul: I will start scanning it and when I'm done I will send it. feel free to remind me in a bit if I haven't. I do think it's probably cooler that you have it.

I will try...not sure why it's italics...to get those numbers tomorrow. Appreciate the help figuring out what I have.

Doc: yeah, I was using mute as a catch all sorts. right now it's wired to b+ and b- but no actual off abilities..

and I hear you on the 5965. all advice by you folks will be followed.

I like the idea of using DC link caps, as I'll be building the case to suit the build. Any thoughts on upsizing to maybe 200uf each? I am a believer in ps capacitance in general...would that help here, and would I need to be concerned about inrush beyond the b+ switch? perhaps soft start needed too?