Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Apexelectric on December 03, 2017, 12:46:39 pm
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I was doing a little clean out of my storage unit when I realized that I had an old portable Pentron HF-400 reel to reel recorder/player. It was given to me by an old friend who owned a music store and was probably in for a repair or for resale. The unit is pretty mint and does not seem to have been messed with at all. It’s loaded with Astron red and yellow capacitors and is basically a 10W 6AQ5 push-pull amp with a couple 6” onboard speakers.
There are also 5 other tubes, 6SL7, 6X5, 6V6, 12AU7 and a 6J7. There’s not much info to be had on it as far as a readily available schematic but was considering trying to transform this into a simple Guitar amp and utilizing the cabinet, speakers, and onboard volume and tone controls.
My big question is whether or not it worthwhile utilizing the old film capacitors and resistors if they bench test ok or if these are just bound to fail. There is always lots of talk regarding the mojo associated with some of these components and the counter opinion of the reliability and sound quality of a degraded 60 year old component.
Is this a gem in the rough or an exercise in futility?
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I'm not sure how likely it is for those caps to be usable, but if you're making major changes you'd re-solder and maybe get different values anyways... they'd be easy/cheap to replace.
If these caps are highly desirable, I'd personally consider selling them and building something with cheap modern parts, but that's personal preference.
If you do want to use them you need to check for DC leakage. A regular capacitance meter probably doesn't do this automatically.
Also, the capacitance value may have drifted (or not been as labeled in the first place) but that doesn't mean they're unusable-- arguably, the "mojo" of vintage parts is due to (slightly) incorrect values.
I think carbon comp resistors are likely to change value over time, especially due to heat or humidity, but if they're close to what you need they'll continue to slowly drift over time and this shouldn't be a catastrophic issue. Capacitors leaking DC could cause serious trouble though.
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A schematic for the smaller 1956 Pentron is available.
I bet it is much the same, except the push-pull audio stage and driver. Does have 6J7(!), does have 6V6 as bias oscillator (also audio output, unlike yours).
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If it's like the Webcor stuff, that will make a fine amp. I just used the transformers and cabinet for mine. I found the pots to be a little sticky after all those years and at 2 bucks and change, used news ones there too.
(http://loafinshed.com/CJ/pic13.jpg)
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Just was unsure if I should go through the trouble to save the Astron red and yellow caps for another higher end project or if they’re not worth the trouble. Haven’t had any experience with them. They look like new so I thought it would be worth asking before I started tearing in to it.
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PRR,
Thanks for the schematic that seems pretty close except for the output like you said.
Here’s a shot of the components and cabinet.
John,
Nice conversion.
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Wow, the guts of that are way better than anything I've ran into yet with the Webcors! I'll bet that one was hardly used at all.
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Yeah, it’s pretty mint. I almost feel bad about repurposing it. But it’s better to give it a second life being used than sitting in a storage shed taking up space.
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Yeah, it’s pretty mint. I almost feel bad about repurposing it. But it’s better to give it a second life being used than sitting in a storage shed taking up space.
I'm curious if you ended up building something with this. I just ended up with one of these, and I'm starting to look at what to do with it.
I found a schematic for this model, I believe, but the schematic says it has a 5879 pentode instead of a 6J7. Apparently they switched tubes at some point?
Either way, I'm starting to look at circuit options, so I'd love to hear if you ended up with something.
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It’s still on the shelf. I decided to put my time into other projects. Haven’t done anything with it yet.
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> a 5879 pentode instead of a 6J7
5879, 6J7/6SJ7 are very similar (except the obvious pin-out and bottle).
6J7 is an *OLD* type, well-regarded as an audio tube (1930s-1940s broadcast gear is full of 6J7 except they called it 1620). 6SJ7 is the same guts but grid-cap moved to base. The world used to be filthy with 6J7 but eventually supply dwindled (not for DIY quantity but you might not want to commit a new design); also fashion changed to smaller equipment and lower-price mini sockets. 5879 is a hair smaller guts in much smaller bottle but will generally do anything the 6J7 will do and probably without circuit change.
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Just was unsure if I should go through the trouble to save the Astron red and yellow caps for another higher end project or if they’re not worth the trouble. Haven’t had any experience with them. They look like new so I thought it would be worth asking before I started tearing in to it.
Every red or yellow Astron I've ever seen leaks lots of DC. You can try them but they will likely also leak. Might be fine for guitar tone controls so you could ebay them but otherwise I wouldn't use them myself.
Greg