Tube amp questions

[Gwcoty]

Didn’t see a place for tube amps so I’m trying here. The amp circuit in question is attached, I’d like to hear some thoughts on it. I’ll probably substitute the 2a3s for 45s, and wire it for a an 80 rectifier.

[old_tv_nut]

I'd think you'd get a lot more action on this over in AudioKarma.

[Gwcoty]

I’m not familiar with that but I will check it out!

[maxhifi]

This amplifier will cost a small fortune to build, and perform quite poorly by modern standards. I would not build it as it is designed, unless you really want to hear what a public address system sounded like in the early 1930s.

[JohnCT]

Originally Posted by maxhifi

I would not build it as it is designed, unless you really want to hear what a public address system sounded like in the early 1930s.

LOL! Agree.

If anyone wanted to build a tube amp, there are a lot of recent designs out on the web, plus you can't go wrong with copying any Williamson design or a Fisher, Scott, Marantz etc.

Also, RCA tube manuals (available in repro and used on ebay) have all sorts of tube projects in the back, and every edition always had one or more hi-fi amps. Any of those would work fine.

The magic is in the output transformers though. You have to source either top notch vintage iron or spend the money and get real good current production.

If I wanted to build a tube amp, I would look for a *true* basket case (rusted outside in a dump, etc.) Fisher or Scott 7591 amp and grab those output transformers and go from there.

John

EDIT: you don't need to buy a copy of the RCA manual. Here is a link to download it for free!

https://archive.org/details/RCA_RC-30_1975

Here's two, a 15W and a 50W amp




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[zeno]

I would NOT cut up an old PA unless it was total junk. I use a Fisher
KX-200 with 7591A outs. I doubt you could improve it. You also
cant buy anything new that will touch it for under a grand at least.

If you want to build for fun think about using odd ball tubes that
can be MUCH cheaper. See the prices of 7591a, 6CA7, 6L6 etc.
Dont worry about brute wattage. With good SPKRS 5-10 watts / channel
give room filling sound. Building tube amps is a pretty big hobby. I bet
there are tons of sites for it. Just start small & if you run into anal orifices
just ignore them. They are out there.

73 Zeno
LFOD !

[maxhifi]

On second thought, I noticed.the brand is National/Dobro, which likely makes this a very very early guitar amp. I've never seen a prewar electric guitar amp before, but in this case the difficulty will be finding appropriate transformers. I would also stay with the 2A3, because it puts out double the power of the 45, and you can find new Chinese ones to save money.

[JohnCT]

Originally Posted by zeno

I use a FisherKX-200 with 7591A outs. I doubt you could improve it.

Yep, the Westinghouse 7591/7868 tube is one of the best sounding output tubes ever made. I have a Scott 299D and a Fisher X-202-B that uses the 7591 outs. Somewhere, I have an X-100-C which uses the 7868. Even though the 7868 is now back in production, it's really crazy priced. Best plan for that would be to rewire for the 7591A.

If you want to build for fun think about using odd ball tubes that
can be MUCH cheaper. See the prices of 7591a, 6CA7, 6L6 etc.

True, those tubes are a bit pricey, but the fact that they are still in production and likely will be forever is a good thing as well.

Dont worry about brute wattage. With good SPKRS 5-10 watts / channel give room filling sound.

Yep. There are some excellent vintage speakers from the 60s that just sound wonderful and are ridiculously efficient. My dad had a pair of ported 3-way 15" Universities that were astounding. I'd love to have a pair today.

My biggest tube amps are my pair of Fisher X1000s which use the EL34s. Right now, I'm running 6CA7 big bottle Russian tubes, but I also have a couple of sleeves of NOS 8417s that I can use.

The X1000 is a beast, but I'm running it through some Legacy Classics tower speakers which are a bit of a current hog. At anything up to and including 60% in a large room, it's a nice combination when I tap them on the 4 ohm lugs. I have a pair of mono SS amps that are very compact, and I've been thinking about using one attached to each speaker and bi-amp them leaving the Fisher to run the mid range and up.

John

EDIT: I found the Universities my dad had. They were 12", not 15" (I was smaller then so they looked bigger). These had amazing sound.

https://www.ebay.com/itm/Vintage-Uni...AAAOSwyVdeQD5J




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[JohnCT]

Originally Posted by maxhifi

On second thought, I noticed.the brand is National/Dobro, which likely makes this a very very early guitar amp.

Good catch. Here is an article on it:

https://www.vintageguitar.com/20982/...l-dobro-6107a/

As an audio amp, probably sounds like crap. This also uses an electrodynamic speaker.

John

[maxhifi]

Originally Posted by JohnCT

Good catch. Here is an article on it:

https://www.vintageguitar.com/20982/...l-dobro-6107a/

As an audio amp, probably sounds like crap. This also uses an electrodynamic speaker.

John

Yeah but all guitar amps sound like crap if you use them full range. That's too cool, there have got to be very few surviving nearly 90 year old guitar amps! Nice find on the article! I'm surprised only 6W from a pair of 2A3

[JohnCT]

Originally Posted by maxhifi

I'm surprised only 6W from a pair of 2A3

B+ at 220V would limit it's power a bit. Or, they didn't want to drive the outputs too much. It would be many years before distortion would be a *good* thing for guitar amps..

John

[maxhifi]

Originally Posted by JohnCT

B+ at 220V would limit it's power a bit. Or, they didn't want to drive the outputs too much. It would be many years before distortion would be a *good* thing for guitar amps..

John

220V appears to be downstream of the field. I bet the field drops a good 50-100V, so B+ is probably in the 300V region as you'd expect it to be. That votage divider resistor in the power supply is sure old school! I really do wonder how it sounds.. 1930s guitar music is definitely interesting in it's own right.

[JohnCT]

Originally Posted by maxhifi

220V appears to be downstream of the field. I bet the field drops a good 50-100V, so B+ is probably in the 300V region as you'd expect it to be.

You're right - the 220V is indeed after the field coil. The output of the field coil (220V) is drawn closer to the rectifier, so that's where I goofed. Not all that clear on my phone, but on my desktop is much easier to see.

John

[mr_rye89]

This is very interesting to say the least. I'm not about to poo-poo building an amp designed in the 1930s because I use one every day. My homemade stereo amp is loosely based on the Western Electric 91A and makes 8 watts per channel. The biggest issue I see is the interstage/phase splitting transformer, But I'm sure Lundahl makes one though.

I'd also drop the first stage,I don't think it needs that much gain.

[JohnCT]

Originally Posted by mr_rye89

I'd also drop the first stage,I don't think it needs that much gain.

Makes one wonder about how much signal a 1930s guitar pickup would have?

John