Carverfest Tube Amp Technical Discussion

12th Annual CARVERFEST 2018 September 5th - 19th
Morris
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Re: Carverfest Tube Amp Technical Discussion

Post by Morris » Tue Oct 09, 2018 10:42 am

angelod307 wrote:
Tue Oct 09, 2018 3:30 am
hello. so I have a request, i want to see some more build photos from everyone.
I have a photo gallery I put together. It might help.
https://www.flickr.com/photos/160853170 ... 8251925912

angelod307
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Re: Carverfest Tube Amp Technical Discussion

Post by angelod307 » Wed Oct 10, 2018 6:36 pm

Hi, I was looking to see what looks great and perhaps things to avoid when wiring.
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Re: Carverfest Tube Amp Technical Discussion

Post by mbskeam » Wed Oct 10, 2018 7:04 pm

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Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Sat Oct 13, 2018 7:19 pm

I've been waiting for a response from Bob about the schematics. I contacted him on Tuesday and he said he was on travel and didn't get the updated copies via e-mail. So I sent him copies to his phone. I still have not gotten any response to whether they are suitable for release or not. I was talking to Wayne (WRF) this afternoon and he said we should move forward with getting them released, so we made some final edits per his request and sent them over to Greg at Carversite. Greg is putting together an assembly instructions packet, so maybe it will help save some time in getting that completed.

I think this is a pretty busy time for Bob right now. Sorry for the delays on getting schematics published, but we didn't think it would take so long to get a final review of them.

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Re: Carverfest Tube Amp Technical Discussion

Post by angelod307 » Mon Oct 15, 2018 2:03 pm

Hi, that’s exactly what needs clarification with lotsa photos as set examples and even drawings of layout is great. Thanks guys.
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UncleMeat
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Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Mon Oct 15, 2018 3:16 pm

The schematics for the 275 are uploaded to thearversite.com Wayne asked me to remove annotations of the grounding from the drawings. He wanted them to be simplified and so there won't be any conflict with the supplied build instructions. Good luck with your amp builds! To those who own CF amps, if you can't download the schematics from Carversite, just let me know I'll e-mail you a copy.

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Re: Carverfest Tube Amp Technical Discussion

Post by angelod307 » Sun Oct 21, 2018 2:25 am

I need a copy cause I can not log on to the other site.
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Robert R
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Re: Carverfest Tube Amp Technical Discussion

Post by Robert R » Sun Oct 21, 2018 1:02 pm

Angelo (and anyone else who has been away from the Carversite forum for a while) - You just need to get a new password - they all were "reset" when the forum changed over. Your last visit of record was three and a half years ago!

Called Greg about this and he will send you a new password, or re-install your old one. In fact, we talked so long that he might have already done it, while we were yacking.

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Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Tue Oct 23, 2018 7:07 pm

UncleMeat wrote:
Sat Oct 06, 2018 3:30 pm
The schematics were sent to Bob on Thursday. Bob has not responded yet, Jim said that he's out of town over the weekend so he probably won't be getting back to me until next week.

I've been looking at the schematics for a few days and a concern over which type of tubes that can be used in this amplifier has come up. The plate voltage is 685VDC and that's pushing the maximum for a KT88 tube. Looking at tubes like the Svetlana 6550, the design maximum plate voltage is 680VDC and tubes like the RCA 6L6GC have a maximum plate voltage of 500VDC.

Going by the datasheet maximums, the stock 275 amp is not designed to run anything but KT120, KT88, and SOME 6550, but not all.

If you have 6550, EL34 or 6L6GC tubes to try I suggest you check the datasheet for your tubes first, or wait to talk with Bob about what changes may need to be made to the amp's B+ so it will work with those tubes.
So we've been discussing this more over on carverstereoforum.com and Jim spoke with Jordon about it. I was also speaking with Kevin about the ability for the 275 to run a higher plate voltage than is listed as a 'design maximum' for various tube types like the 6550 and 6L6GC. Jordon confirmed via Bob that these tube types should work without failure despite having a lower plate voltage maximum.

Talking with Kevin about it, he says that Bob tested the KT-120 at B+ voltages much higher than it's design maximum (upwards of 1200VDC), before it failed due to arc-over. We think the reason the lower-spec tubes will work is because within the design of the tube amplification circuits, the current is kept very low; so the cathode-screen-plate will not see enough electron flow to form an arc.

So for the most part, the vintage octals (that are generally compatible with KT-88 tubes) should work. I wouldn't say that every tube will work, 6L6G or 6L6GB may not work; it really depends on the tube because the amp is operating outside of what the design spec's say are 'possible' for plate voltage.

It's a "Bob" amp, so what else would you expect??!! 8)

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Magnaryder
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Re: Carverfest Tube Amp Technical Discussion

Post by Magnaryder » Wed Oct 24, 2018 12:52 pm

I have some 1940s vintage 6L6s that have plate maximum of 440 volts. Bob thought they might fail in the amp. I have not tried it yet but most likely they’ll stay in the boxes. Ruining 80 year old tubes someone else might enjoy would suck.

ray

Btw, it’s my understanding one amp owner purchased much larger output transformers for his amp build. The modded amp will compete with a stock amp to determine whether there is anything to be gained from the mod. More details when they surface.
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UncleMeat
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Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Wed Oct 24, 2018 6:24 pm

Magnaryder wrote:
Wed Oct 24, 2018 12:52 pm
I have some 1940s vintage 6L6s that have plate maximum of 440 volts. Bob thought they might fail in the amp. I have not tried it yet but most likely they’ll stay in the boxes. Ruining 80 year old tubes someone else might enjoy would suck.

ray

Btw, it’s my understanding one amp owner purchased much larger output transformers for his amp build. The modded amp will compete with a stock amp to determine whether there is anything to be gained from the mod. More details when they surface.
Yes, that would be a shame if a nice pair or set of vintage tubes got nixed. Bob said 6L6GC's should be fine, to that's probably the lowest we should push it. Most vintage 6L6GC, 6550 have a plate voltage maximum of around 500VDC. I would look for a datasheet that is specific to your tube in both brand and vintage to check on the exact value. Look at the "Class AB" maximum ratings, and the "design maximum" numbers which should be at the top of the datasheet.

I was speaking with Bob about the size of the output transformers at CF. We were discussing how the amp could deliver adequate bass with a smaller OTF. The concern over it was the possibility of the transformer 'saturating' with the current delivery requirements, as many expect would tend to happen. He says it's not a problem, because the amp design accounts for the transformer size.

The winding ratio in the stock transformers, which is custom designed, assures it will work correctly because the step down ratio is tailored to the B+ and signal to the expected speaker loads. A smaller transformer is preferred (within limits), because it reduces the inductance. A reduced inductance through the transformer results in better bass control, not worse.

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Magnaryder
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Re: Carverfest Tube Amp Technical Discussion

Post by Magnaryder » Thu Oct 25, 2018 10:15 am

Russ wrote:
I was speaking with Bob about the size of the output transformers at CF. We were discussing how the amp could deliver adequate bass with a smaller OTF. The concern over it was the possibility of the transformer 'saturating' with the current delivery requirements, as many expect would tend to happen. He says it's not a problem, because the amp design accounts for the transformer size.

The winding ratio in the stock transformers, which is custom designed, assures it will work correctly because the step down ratio is tailored to the B+ and signal to the expected speaker loads. A smaller transformer is preferred (within limits), because it reduces the inductance. A reduced inductance through the transformer results in better bass control, not worse.
I guess this particular attendee will let us know in time, I for one am interested in the results. Bob knows better than the rest of us about his design parameters.
When testing my 2011 amps he was surprised how flat and extended the frequency response of the amp looked on the scope, even with 10 watt transformers. 18 watts per channel 7hz to way beyond my hearing. I’m curious to see how the new amp tests out. I think Greg Garska commented he put his on the bench, but I’ve yet to see any concrete numbers and/or graphs. Might be nice to see them.

ray
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Re: Carverfest Tube Amp Technical Discussion

Post by MRHP » Thu Oct 25, 2018 7:33 pm

Ed "sea" and I were talking about the small transformers. After he put one amp together and tested it against a 300x2 on his Plat's he was very impressed with the CF 275. Ed said it put out more volume than the 300x2 at the same volume setting on his preamp.

Ed my want to chime in if I got any above info incorrect.

That being said I'm very curious to hear about the results of larger transformers.
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UncleMeat
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Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Thu Oct 25, 2018 8:02 pm

I think some of Bob's best design work is in this stereo amplifier. No frequency response testing really necessary IMO, just listen to it!

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Magnaryder
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Re: Carverfest Tube Amp Technical Discussion

Post by Magnaryder » Thu Oct 25, 2018 10:25 pm

UncleMeat wrote:
Thu Oct 25, 2018 8:02 pm
I think some of Bob's best design work is in this stereo amplifier. No frequency response testing really necessary IMO, just listen to it!
I’m certain our amp is the result of his latest thinking on tube amp design, that’s not the point. You can’t know where you’ve gone if you don’t know where you started. Benchmark everything otherwise any conclusions drawn are just spurious data😎

ray
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