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Old 7th April 2011, 09:54 AM   #1
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Default Uses for Cheapo 6CB5

I have a bunch of cheap 6CB5s from an AES buy I made a couple of years ago. I also have some sets of P-P transformers in the 3k-3.5k impedance range. This sort of whispers "triode P-P" to me. The screen is normally limited to 200V in this tube, but it'll be tied to the plate so that it won't necessarily draw power it shouldn't, especially if I use a resistor from screen to plate to make sure of that. The screen support posts of these 6CB5s also have a sizable radiator tied to the support posts. I've heard this tube looks/sounds good in triode mode. Yea or nay?
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Old 7th April 2011, 11:29 AM   #2
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hey-Hey!!!,
I have an amp underway with 6CB5A power tubes. 5k a-a output, and 270V B+. Going to run them Class A pentode with g2 around 110V. EL84 LTP for the front end...no triodes in sight.
cheers,
Douglas
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Old 7th April 2011, 01:46 PM   #3
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I have seen reference to an SE amp (possibly on this forum). The builder claimed that the 6CB5A sounds as good as a 300B. Can't find it now, I am posting from a cell phone. I haven't tried that experiment yet.
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Old 7th April 2011, 02:04 PM   #4
colinB is offline colinB  United Kingdom
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Is this the thread?

SE 6CB5A amp with 6N7 driver

Colin.
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Old 7th April 2011, 06:12 PM   #5
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Quote:
Is this the thread?
Yes, that's it. 400 volts on the plate in triode mode with a 1K cathode resistor.
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Old 7th April 2011, 07:31 PM   #6
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Hi!

I wrote about that 6CB5A amp.
More information about amps with this tube can be found on my blog. Click on the 6CB5A label in the sidebar and you'll get all posts about this tube.
I mainly used it in SE but also built a PP (triode strapped)

Link to all 6CB5A related posts on my blog:

VinylSavor: 6CB5A

Feel free to ask any questions about it here on the forum.

best regards

Thomas
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Old 7th April 2011, 07:37 PM   #7
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I have a set of 3.5k P-P transformers used in a PPP 6BQ5 amp (Baldwin organ), a set of Electra-Print transformers intended for something like a pair of 6B4Gs, and a couple of Amplimo toroidal output transformers with ~3k impedance. I'll probably try the 6BC5s with the Baldwin transformers, as they are suppossed to be capable of some decent output power. I will also be less worried about unbalanced primary current with the E-core transformers. The Electra-Prints will be reserved for use with some 6S4S triodes and my own take on some isolated switching supplies for the directly heated cathodes.

For the 6CB5s, I may use some kind of single ended interstage transformer or a choke-loaded driver to take advantage of the extra voltage swing afforded by that approach. The trioded 6CB5s have a rather low mu (~3.8), so they will need some volts to drive them to best advantage.
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Old 7th April 2011, 07:48 PM   #8
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Hi!

If you want to go Class A NFB PP 3,5k would be quite low. The 6CB5A has an rp of about 1k triode strapped.

Best regards

Thomas
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Old 7th April 2011, 08:21 PM   #9
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That's interesting. I also have some ~6k Fisher P-P iron (Fisher 500B and 500C) that would be more suitable in that case, and certainly higher quality than the Baldwin transformers. If I can indeed run the trioded 6CB5s at 400V B+, there woulldn't be much of a power penalty from running at the higher primary impedance.
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Old 10th April 2011, 09:36 PM   #10
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More thoughts - I've been leaning toward using a trioded high transconductance pentode like the 12HG7 for the input stage (Stu would say use a D3A, but I want to make this amp completely out of cheap TV tubes to prove a point), with a SE interstage transformer to drive the trioded 6CB5A outputs. I have a pair of Fisher 500B P-P output transformers on hand that shouldn't muddy up the sound too much. The SE interstage will serve both as choke load for the input triode and as a driver transformer. This may be my introduction to winding my own interstage transformer. I have on hand what I think is a suitable bobbin. I need to get out my calipers, but it looks like it's sized for either an EI75 or EI100 lam set. Once that's decided, the next step is to set up the equations and check out the turns needed to support a ~30ma bias current, ~12kG peak flux density, and enough inductance so my lows are ok. No fancy stuff this time around - silicon steel, butt-stacked construction. Another candidate for the input tube might be the big pentode inside the 6AG9, but then I'll have an unemployed triode sulking there beside it looking for something to do...
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