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6p3p tube amplifier kit
6p3p tube amplifier kit
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Old 23rd July 2016, 10:35 AM   #221
LDA is offline LDA
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Join Date: Apr 2014
Hi folks, first post but I've been a pretty heavy lurker up until now.
I bought one of these kits a while ago as a first build, and have to say it works beautifully! I've upgraded it a bit here and there, backup SS diodes, 6N1p-EV tube, alps pot(stock one had no taper then full volume...), made feedback loop adjustable, pair of hum balancing resistors, etc.
I did have it paired with a set of Akai SS-110 speakers I found on the side of the road, which sounded pretty nice but recently got given a pair of 70's Kef Concertos, which are just incredible.
Unfortunately it has trouble driving the concertos, as they're not as sensitive, and had to remove (some of) the feedback to get a useful volume out of them, at the cost of the sound quality.

Raindance I was interested in trying your circuit to switch to pentode mode to hopefully get it sounding a bit better/louder.
Could you explain how SEP compares to SET mode? Like how the sound compares and power differences. Did you test it with a scope? Unfortunately I'm a noob and poor so don't have a scope

Cheers!
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Old 23rd July 2016, 08:28 PM   #222
raindance is offline raindance  United States
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Join Date: Jul 2010
Greetings. I'll need to check mine again. I think it only pushed about 3 watts before clipping in stock form. It should get 5 or 6 watts with my mods, but the transformers and the power supply are limiting factors. I'll measure it and let you know. I don't like the sound without feedback. Feedback helps dial out some of the bandwidth limitations of the transformers. However it doesn't really have enough gain to deal with the feedback properly. The good thing about this is that it's pretty darn stable. Not enough gain to get you into trouble. I'm not sure if my last schematic omitted the cathode bypass cap on the input stage, but I ended up putting them back in to increase gain. I'm on my phone, so can't look it up easily right now.
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Old 27th July 2016, 01:23 PM   #223
LDA is offline LDA
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Join Date: Apr 2014
Thanks Raindance! Yeah I definitely agree about the amp sounding better with more feedback. I hear all sorts of scare stories about negative feedback but it certainly makes sense scientifically and to my ears as well
Hopefully I can squeeze an extra few watts out of it with pentode mode, perhaps with a bit more distortion to start but with a few dB more feedback it should definitely be sounding better and driving these speakers better too
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Old 5th March 2017, 05:19 AM   #224
secondhandni is offline secondhandni  Australia
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Join Date: Jan 2011
Hi all, first post in a while, and first time I have attempted to build anything high voltage before. I have a basic understanding of electronics and have had valve amps for years, I decided to build a siliconray amp as a gift for my brother in law and wanted to share progress.

About 5 hours in now and am taking a different approach to normal construction methods. I have decided to build in order of access rather than doing power/heaters first.

There a very small amount of space in this chassis and i can barley fit my hands in there. I decided to use a ground bus system in the center of the chassis and work my way from the socket wiring forwards.

Click the image to open in full size.

trying to keep it as neat as possible. only mods so far are 330k resistors for feedback loop. these can be easily changed if I decide they are not up to tastes.

I will be replacing the tubes with 6N1P-EV driver and 6L6G NOS power tubes.

I will keep you posted once its finished.

1 question for those using 220V. the transformer is 110 0 110. how do i bridge for 220v?

Last edited by secondhandni; 5th March 2017 at 05:21 AM.
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Old 7th March 2017, 01:37 AM   #225
raindance is offline raindance  United States
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Join Date: Jul 2010
330k for feedback is about the same as none at all. It'll have too much gain and have floppy bass. That value is far too high.

The mains transformer probably needs to be series connected for 220 volts. Certainly not parallel, that would only work for 110 volt and will effectively double all the internal voltages.

By the way the power transformer is underrated. It runs uncomfortably hot and I don't recommend leaving this amp running unattended ever. I've tried a step down transformer to lower the mains voltage a bit but it didn't help. I think a separate transformer is probably needed for the output tube heaters.
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Old 7th March 2017, 04:32 AM   #226
secondhandni is offline secondhandni  Australia
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Join Date: Jan 2011
Thanks for the feedback radiance, can you suggest a better value for the feedback loop?

I have checked the transformer and you are correct if i connect it in series it will be 220V The amp will be used in Korea and they run 220V 60Hz. but is being built in Australia 240V 50Hz.

I know when the input power is reduced to the transformer it will mean the bias will be lower for the tubes, but you have me concerned about the power transformer overheating.

can I drop the bias for the tubes enough to reduce the load on the transformer? I was thinking to run 6l6GA's with lower power requirements and a 30ma bias.

I am not sure sure what the plate voltage will be fore the amp at 220v and 240V.
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Old 7th March 2017, 09:46 AM   #227
secondhandni is offline secondhandni  Australia
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Join Date: Jan 2011
more research suggests 6l6GA's will require 57ma bais and with a 300v plate this will mean a dissipation of 16 watts. the calculator you mentioned gives me 6l6gc at 90ma bias @ 300v with a dissipation of 26 watts.

would this mean running 6L6GA be pulling 20 watts less load from the transformer.

Have I got this incorrect?
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Old 7th March 2017, 10:17 AM   #228
raindance is offline raindance  United States
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Join Date: Jul 2010
So at 240 volts everything will be a little high in terms of voltages. I have suggested before that an APC Line-R unit will actually provide the right amount of step down if you force it into overvoltage mode. You should try one of those, but only if you have issues when you measure everything.

Bias current will have little effect on the transformer heating. It is only a few mA. It is the heaters that draw a lot of power. Your ideas about bias indicate that you don't understand single ended amps. 30mA would be good for half of a push pull circuit. In this circuit the tube conducts for the whole cycle and needs to be biased deeper. So if you think 30mA you mean 60mA

I think you should get it running with the supplied tubes. The 6P3P is a great sounding 6L6 equivalent and the circuit as designed and with my pentode mods will accomodate it or a 6L6 or an EL34.
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Old 8th March 2017, 09:22 AM   #229
secondhandni is offline secondhandni  Australia
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Join Date: Jan 2011
Thanks again Radiance,

i will get cracking this weekend and finish the amp. I have a stock of

6L6GA
6L6GC
6L6GB
6P3S
6V6
6C3N
6C3N-E
EL34

and a few others for the power tubes. Also have every driver tube known to man. I have about 2,000 tubes lying around and have been collecting for years. mostly whenever I see an electronics store in the countryside I stop and ask if they have any old radio tubes. 95% of the time they hand over a box for free.. best was a NOS box of ken rad 6l6G's!

by tube collecting hobby has only recently turned into building stuff with them.
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Old 8th March 2017, 09:32 AM   #230
RobFreak is offline RobFreak  Denmark
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Join Date: Dec 2009
Quote:
Originally Posted by secondhandni View Post
Thanks again Radiance,

i will get cracking this weekend and finish the amp. I have a stock of

6L6GA
6L6GC
6L6GB
6P3S
6V6
6C3N
6C3N-E
EL34

and a few others for the power tubes. Also have every driver tube known to man. I have about 2,000 tubes lying around and have been collecting for years. mostly whenever I see an electronics store in the countryside I stop and ask if they have any old radio tubes. 95% of the time they hand over a box for free.. best was a NOS box of ken rad 6l6G's!

by tube collecting hobby has only recently turned into building stuff with them.
I could use some EL34. Which ones do you have, sorry for asking such a vague questions, i bet you have TONS! ;-)
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