6CB5 in triode PP?

[deicide67]

Something like this maybe?

No values because I don't know if these are the tubes I will use yet. I added the gain stage on the input in case the first triode of the 6GF7A is not sufficient. Should those stages be cap coupled?

Thanks!

Blair

 

[deicide67]

Does the 6GF7A I am proposing to use have enough voltage swing to drive a pair of 6CB5As to full output, or is a gain stage necessary to accomplish full output.

Something like a high powered "El Cheapo" with only three tubes.

Thanks!

Blair

 

[deicide67]

I have little fingers around the house, so I'm going to try to keep the top caps under the hood. There is adequate venting above and below the bulbs in this old chassis from a Proceed preamp. The Ecdors will fit in here as well as long as I stick with the 60W outputs.

You think I will need a "whisper fan" to circulate around the output tubes?

Is there enough room to build an amp in the chassis without too much crowding? This is one of the reasons I want to use the 6GF7A as a single tube driver.

Thanks!

Blair

 

[deicide67]

How much voltage swing is necessary to drive a pair of these 6CB5s? The 6GF7A is essentially a 13EM7 in a novar bottle. First section gain is 64 and second section is 5.4.

Will I need an additional gain section in front of these 6GF7As to get full drive voltage?

Thanks!

Blair

 

[deicide67]

Forch, I saved all the original notes. Loadline (attached) Do keep in mind that this particular project never got built, so it's very much a preliminary design.

With an Rl= 635R/phase, you'll need ~2.54K (P-2-P) A 3K3, 60W Edcor should be just fine here, and the somewhat lighter plate load will drop the output a bit, and maybe the distortion as well (though it doesn't look like you're gonna get very much of that anyway).

Even though these H3 estimates are rough, and high-side errors, they do correlate somewhat to sonic performance. However, in the final analysis, you still have to spend some time listening to the open loop performance to see how that distortion sounds, and what's likely to be necessary to correct for those sonic defects. If the 6CB5 is like the 6BQ6, all it'll need is enough gNFB to take the "edge" off. There shouldn't be any of that "pentode nastiness".

I may have found a pair of 1650T Hammonds which are 1.9K and 120W. Is that way too low for a pair of the 6CB5 tubes?

Thanks,

Blair

 

[jazbo8]

I thought Miles already answered all your questions on the drive requirement and the plate load.

 

[deicide67]

Hi,

He did by stating that the optimal load would be 2.54K, but 3.3K would suffice at the cost of a little power. 1.9K is less difference to 2.54K than 3.3K, so I am asking if it would be an OK plate load to use 1.9K vs. 2.54K.

Will the 1.9K load cause excessive distortion?

So, yes, miles did answer the question.

 

[jazbo8]

Hi,
Will the 1.9K load cause excessive distortion?

No, the distortion would be a little bit higher but not what you would call excessive, especially at low-medium playback volume, fully crank'd, you will hear it - or not, as it would be so darn loud, you may have to cover your ears

 

[deicide67]

Thank you. That's what I'm asking more less. Assuming I can get the output iron at a good price.

If not, I will suck it up and wait 6 weeks for Edcor

 

[jazbo8]

Just curious, what speakers are you using, do they require all that power?

 

[deicide67]

~88db.

40W or so is fine for casual listening, but anything with serious bass transients needs a brass pair to grip these bass drivers.

 

[jazbo8]

Nice looking OBs! What are the drivers? So the 6CB5 will drive the bass speakers?
I have only one Eminence Beta 15A per side, driven with a 100W Class D amp (shameful I know), perhaps it's time for an upgrade...

 

[TonyTecson]

Hi,

He did by stating that the optimal load would be 2.54K, but 3.3K would suffice at the cost of a little power. 1.9K is less difference to 2.54K than 3.3K, so I am asking if it would be an OK plate load to use 1.9K vs. 2.54K.

Will the 1.9K load cause excessive distortion?

So, yes, miles did answer the question.

as a rule of thumb, i always prioritise SQ over power ratings.....
in your case i will choose 3.3k, you can connect the 8 ohm speaker to the 16 ohm tap is you wanted more power...

 

[deicide67]

nope. They are fully passive and dynamic as almost any box speaker I've heard. The bass drivers are Acoustic Elegance IB15, midrange Peerless 830883, and the BG Neo3PDR with back cup removed for the tweeter.

I'm playing with the sweeps for fun because I cannot afford the tubes for my big amps yet

 

[deicide67]

as a rule of thumb, i always prioritise SQ over power ratings.....
in your case i will choose 3.3k, you can connect the 8 ohm speaker to the 16 ohm tap is you wanted more power...

I will see about the pricing difference between the Edcors and the guy with the Hammonds.

We will see. I'm actually curious about the sweep tubes. I've never tinkered with them before.

 

[jazbo8]

Cool! Let me know how the 6CB5 works out on the LF, I have not tried driving the bass on mine with a tube amp yet, since they require good control as you said.

 

[deicide67]

Don't feel shamed. I use a Emotiva 175W amp to drive my speakers right now. I'm looking for tubes that will nail my drivers with the same authority

My other project is a PPP KT120 amp. That should do it!

As for the 6CB5, I'm thinking that maybe I should go with monoblocks using PPP since I have two power transformers.

Again, speculative on the hammond pricing.

 

[TonyTecson]

I will see about the pricing difference between the Edcors and the guy with the Hammonds.

We will see. I'm actually curious about the sweep tubes. I've never tinkered with them before.

the OPT is the single most expensive item in an amp, getting bigger(heavier) than you need makes for better bass...

 

[deicide67]

I gotcha I've built a few amps, just nothing with sweeps. I'm familiar with output iron and cost/size/performance.

 

[TonyTecson]

at a few dollars a pop what is not to like about sweep tubes?
though the 30 watters have moved up in price....
in my case i resist all temptations to build with hyped tubes, the costs are simply silly imho..