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Old 21st September 2011, 05:39 PM   #31
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Alain Poitras View Post
Personnally, I will not use a bypass capacitor on the driver tube to make an additional local feedback that make the distortion much lower than with just a global feedback, but the stage will also have less gain ... The two triodes use for the SRPP can be use a different way too, in a Schade circuit with an additionnal preamp stage for example, to get more gain ... Three differents kinds of feedback together are much better then none at all ... It is like day and night ...
There are so many way to go, none is "best". For example that cascode stage is populater but I find that if you use a "normal" high gain preamp triode and add a NBF resistor the distortion goes lower then I can measure with equipment I have plus the added advantage that I can fine tune the gain of that stage. Fine tuning allows some fine scale adjstmet to the sound b swapping out 10 cent resistors Also use global NFB.

The disadvantage of using local feedback around a preamp tube is that (1) you loose bragging rights on saying how the mega-expensive and rare NOS tube sounds because with local NFB that all are near perfect and (2) the common cathode stage is less stylish but my opinion is that fewer parts (especially tubes) the better


About hum: wait until you are all done. First build the amp using 6.3 volts AC heaters. Yes do the easy thing and elevate the heaters to at least match the power tube cathode. Then listen. Good chance it will be silent. But if not the first test is to run the heaters using a BATTERY. Nothing is more quite. Make an A/B switch and flip between AC and battery. If this solves the hum only then go for DC heaters. But there is a good chance that hum is in the B+ power or magnetic coupling from PT to OPT or even a microphonic preamp tube picking up 60Hz vibration from PT. The battery test will help sort that out.
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Old 22nd September 2011, 02:03 AM   #32
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Just checking in here....(I'm away from my home base)....

That amp (in the thread Building something better with a Chinese SE EL34B Amp DIY Kit turned out pretty well, IMO.
Sounds fine driving ~95dB AudioNirvanas.
Have fun! (It was my one-and-never-to-be-repeated amp with a stainless chassis.)
John
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Old 23rd September 2011, 11:39 AM   #33
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Default Plan of attack

OK then, well, I'll hang fire till the kit arrives and then I'll photograph it, build it, run some measurements and get back to the forum for some tweaking help.

Cheers all, and I'll repost in a coupla weeks!
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Old 24th September 2011, 03:45 PM   #34
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Quote:
Originally Posted by ChrisA View Post
... The disadvantage of using local feedback around a preamp tube is that (1) you loose bragging rights on saying how the mega-expensive and rare NOS tube sounds because with local NFB that all are near perfect and (2) the common cathode stage is less stylish but my opinion is that fewer parts (especially tubes) the better ...
What I mean by "local feedback" is just "no cathode bypass capacitor" ... Less gain but also less distortion ...

I also believe fewer parts is better and capacitors are not very good for the sound, using just high gain triodes like the 12AX7 can compensate for the lack of gain caused by the feedback ... I am not crazy about using penthodes in preamplifiers stages, but if it is better than two triodes stages, why not ...

Last edited by Alain Poitras; 24th September 2011 at 03:47 PM.
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Old 24th September 2011, 06:05 PM   #35
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Alain Poitras View Post
What I mean by "local feedback" is just "no cathode bypass capacitor" ... Less gain but also less distortion ...

I also believe fewer parts is better and capacitors are not very good for the sound, using just high gain triodes like the 12AX7 can compensate for the lack of gain caused by the feedback ... I am not crazy about using penthodes in preamplifiers stages, but if it is better than two triodes stages, why not ...
You are right. But I meant, rather than the cascade to literally use one triode with a feedback resister going back to the grid. With this, you can dial in the exact gain you need in that stage. I built this on a breadboard. I then subbed tubes even putting in a 12AT7 for my 12AX7 and measured the harmonic distortion and eyeballed the wave on the scope. The local feedback resistor holds the gain dead on and hold THD bellow what my older distortion meter can measure. Of course an unbypassed triode has some of this effect but there is an order of magnitude difference (almost literally)
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Old 6th October 2011, 12:54 PM   #36
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Default The kit's arrived!

Rather more swiftly than I would have expected, the kit has arrived. I quick summary of my feelings during the unboxing experience would be 'mixed emotions'. A few pics here https://picasaweb.google.com/aspring...eat=directlink
for any who are interested to have a look at.

Click the image to open in full size.

https://picasaweb.google.com/lh/phot...eat=directlink

The transformers made a mess of the box they were in but appear to be undamaged and pleasingly hefty, so I'm naively hopefull that they're adequate to the task. If there's any tests that anyone can propose to me to check em, please let me know.

Click the image to open in full size.

The tubes are a bit dodgy though! The EL's are a new pair, but the 6n9p's are clearly different and one is potentially a used item. The rectifier is definitely 'pre loved'. I've a pair of 12ax7's here in a spares box, so perhaps a redesign utilising these in the future will be in order.

Click the image to open in full size.

The power switch made me laugh - the label for it reads 'on ' and 'on' apparently they work their amps 24/7 in china...

Click the image to open in full size.

The caps are ok-ish. The two 220uf/100V ones are salvaged from somewhere, but the rest are new... If not of the the values in the schematic. However they are appropriate to use in the given circuit.

Click the image to open in full size.

Then there's the missing parts. Some resistors, 4 speaker posts, and that's about it!

I'll be sending off a complaint about the used parts given the item is described as 'brand new', and with regards to the missing bits.

Hope that's interesting to everyone!

Last edited by aspringv; 6th October 2011 at 12:57 PM. Reason: clarity and brevity are the soul of good communication.
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Old 6th October 2011, 12:58 PM   #37
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Default ruddy pictures are too big

I'll repost them at a later point in smaller size...
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Old 6th October 2011, 04:22 PM   #38
TheGimp is offline TheGimp  United States
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Quote:
Originally Posted by ChrisA View Post
You are right. But I meant, rather than the cascade to literally use one triode with a feedback resister going back to the grid. With this, you can dial in the exact gain you need in that stage. I built this on a breadboard. I then subbed tubes even putting in a 12AT7 for my 12AX7 and measured the harmonic distortion and eyeballed the wave on the scope. The local feedback resistor holds the gain dead on and hold THD bellow what my older distortion meter can measure. Of course an unbypassed triode has some of this effect but there is an order of magnitude difference (almost literally)
One caviat when doing this is you also radically decrease your input resistance. It can drag down the driving source depending on the device driving he input.
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Old 6th October 2011, 05:17 PM   #39
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Quote:
Originally Posted by aspringv View Post
Rather more swiftly than I would have expected, the kit has arrived. I quick summary of my feelings during the unboxing experience would be 'mixed emotions'.
I'm looking forward to the pictures!

How was the chassis? i.e. does it match up with the transformers?

Have fun!
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Old 6th October 2011, 05:46 PM   #40
data is offline data  Australia
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LOL, for a moment I thought the chassis was made from cardboard.

Doh!

https://picasaweb.google.com/aspring...30885357169874

Silly me
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