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Old 23rd March 2007, 02:40 AM   #1
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Default use a 45 to drive a 45 ?

Soooo,

I'm still wrapping my brains around the scratch-built thing. I can read a schematic, modify an exisiting amp, and have a 6V6 DIY amp that *almost* works (shoulda breadboarded first -power supply issues - read: attention to detail lacking ).

My question is this:

I have some decent 50's era OPT's and a few PT's kickign around which will fit the bill for a 45 amp, and a decent stash of 45's, 26's, 27's, and 80's etc, etc . . .

I'm leaning towards building the JE Labs Simple 45/2A3 cause it's you know - simple, BUT, in digging around for info I've seen two things:

1) 45 tubes are loved for their transparency and midrange

2) some of the more impressive amps I've seen use the same tube for the driver and the output tube.

Could you do this with a 45 ? is it just a matter of getting the operating points right on the 'driver' 45. I've searched all over, and can see guys doign this with the other power triodes (845, 211, 300b), but why not the 45 ?

I know the short answer is 'time to break out the 45 data sheet and get intimate', but I thought I'd see what the experts think . . .

thanks !
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Old 23rd March 2007, 03:04 AM   #2
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I'd be hesitant to do this for the following reasons:

1. NOS 45s are evermore rare and hard to find. Preserving the existing stock is important. Why burn up a tube in the driver role when you can burn up a tube in the output stage role?

2. The mission of driving a 45 with 45, or 845 with 845 is to cancel
distortion. The 45 already has such low distortion that this is pretty much fruitless. I dont think you will hear anything but it will "look"
impressive to use more rare tubes I guess.

3. Using a a 45 to drive an 845 makes sense. Because you are using 1.5W of the 845 to drive the grid of the 845 to get 20W out. In this case, the 45 is doing a great job swinging lots of volts at low distortion. This is a case where burning up a 45 as a driver begins to make sense.

-- Jim
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Old 23rd March 2007, 03:06 AM   #3
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hey-Hey!!!,
gain might be an issue. A 5k:16 output TX is ~17:1 step down. Two 45's in cascade will have a combined gain of ~1:13. Your amp will have a gain less than 1.

It gets worse when you go 5k:8R which is a 25:1 step down. Consider perhaps a 26 driving a 45. Still not going to have a lot of gain, but power output won't need a lot of output voltage. A 45 SE won't do 4V rms into 8R, more like 3.5 V rms. A reasonable pre-amp/linestage should be able to deliver a proper signal to the amps.
cheers,
Douglas
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Old 23rd March 2007, 03:39 AM   #4
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Hello

The best wishes you with the project using 45 as driver for 45, Just in case not successful, pls try to use this schema which I believe that you will never care to another amp under USD3000.
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Old 23rd March 2007, 03:42 AM   #5
rdf is offline rdf  Canada
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Quote:
Originally posted by jrdmedford
2. The mission of driving a 45 with 45, or 845 with 845 is to cancel distortion.
It's not a given in any case, effective cancellation doesn't result from simply cascading identical tubes. The driver must be set up to generate exactly the correct level of even harmonic and maintain it across a wide input variance. Since the two 45's will typically be under significantly different operating conditions it's by no means assured.
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Old 23rd March 2007, 04:24 AM   #6
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Default Re: use a 45 to drive a 45 ?

Quote:
Originally posted by bottom_feeder
Soooo,


My question is this:

I have a decent stash of 45's, 26's, 27's, and 80's etc, etc . . .

2) some of the more impressive amps I've seen use the same tube for the driver and the output tube.








this and the power supply, will be the most important aspects, in the final sound.

i vote yes


from here-

http://www10.big.or.jp/~dh/sp/DHWS_2.html



- Through listening, I discovered that the driver's tone contributes the greater part of the amplifier's voice.-

An excellent power tube demands a similar level of quality from its driver. Few driver tubes can sing along in beautiful harmony with a good power tube ."


http://www10.big.or.jp/~dh/index.html#koko1

58
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Old 23rd March 2007, 10:54 AM   #7
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Quote:
Originally posted by rdf


...Since the two 45's will typically be under significantly different operating conditions it's by no means assured.

Yes, ... and the load for the output tube varies with frequency (depends on loudspeaker).

best regards
Andreas
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Old 23rd March 2007, 03:28 PM   #8
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Default food for thought . . .

I appreciate all the input guys : )

to Jim's point, all my tubes mentioned in this thread are pulls from old radios, so not NOS, but I definitely see the point of not burning up a dwindling resource unnecessarily. . .


Doug: my opt's into an 8 Ohm load will be 2.2K:1 - I see the gain issues, and a preamp / linestage would definitely be used. The details there are yet to be decided, but ya gotta start somewhere

tomtt: the 'Sakuma Philosophy' was where I got the hairbrained idea
the power supply will be using a TON of oil caps, and I have some great old chokes which will be put to use also.

Loudspeakers to-be-driven for starters would be a very simple OB design; I have a few kicking around, and a few more 'in the works'. I'm not really expecting earth shaking volume, I basically want to explore the sound of the 45 tube. . .

Regardless, this will be a two-chassis design, so down the road I can experiment with different output stages . . . time to hit the books . . .
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Old 24th March 2007, 02:46 AM   #9
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Default Re: food for thought . . .

Quote:
Originally posted by bottom_feeder
I


tomtt: the 'Sakuma Philosophy' was where I got the hairbrained idea


the power supply will be using a TON of oil caps, and I have some great old chokes which will be put to use also.

Loudspeakers to-be-driven for starters would be a very simple OB design; I have a few kicking around, and a few more 'in the works'. I'm not really expecting earth shaking volume, I basically want to explore the sound of the 45 tube. . .

Regardless, this will be a two-chassis design, so down the road I can experiment with different output stages . . . time to hit the books . . .
yur my hero
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Old 24th March 2007, 03:14 AM   #10
Tweeker is offline Tweeker  United States
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Quote:
the power supply will be using a TON of oil caps, and I have some great old chokes which will be put to use also.
Hey, 300Bs make for nice regulators.
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