Future plan for 300B XLS amp

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IslandPink
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#211

Post by IslandPink »

Just installed Mullard EL86 as pentode drivers on the 300B amp (aka 'experiment' ) . Same load as the 4P1L's - 7.8k .
I put in 470R cathode resistors first, forgetting to allow for the current from G2 and the OA2 - D'Oh ! - than changed to 330R , which is better, and gives me about 260V on the anodes.

So far it sounds very good, well balanced, not much different to the previous C3m drivers to be honest.

I think the treble is not as resolved as with the 4P1L ( DHP ) drivers, which is good - accords with what I found going from C3m to 4P1L . Bass , I'm not sure yet - may be a bit tighter, but not sure bass tone is as good.
At times I think the instrument separation & organisation is a touch better with the EL86 compared to 4P1L . Maybe there were some microphonic effects in the 4P1L or the ceramic caps in the filament supply ?

It's important to me as I want to gauge whether I was ahead with the 4P1L vs. simpler IDHP drivers. This turned out to be a big battle with the filament supplies to achieve that extra quality. As of today ( more listening to be done ) I think I was just slightly ahead with the 4P1L and the last iteration of Andrew's filament supplies. Since we have a further few tweaks to do, I think the DHP will be the way forward in future.

I can however definitely recommend the EL86 which costs about £8 for Mullards - they have a useful anode dissipation of 10 or 11W which makes then comfortable on a 450V supply for 'super' 300B's compared to the C3m which is on the limit .
"Once you find out ... the Circumstances ; then you can go out"
Cressy Snr
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#212

Post by Cressy Snr »

The EL86 looks like a real sleeper valve to me :wink:
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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pre65
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#213

Post by pre65 »

SteveTheShadow wrote:The EL86 looks like a real sleeper valve to me :wink:
Like an EL84 but higher current and lower anode voltage. (so I've read) :)
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IslandPink
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#214 Re: Future plan for 300B XLS amp

Post by IslandPink »

Probably worth putting this info here as it is a 'future plan for 300B amp'
Been pondering a problem with upping the voltage from the Shunt-Reg power supply.
http://www.audio-talk.co.uk/phpBB3/view ... nt#p103681
I would like to go to GK-71 and GM70 SE amp eventually with this. Especially GM70 (coppers) which will need around 700V to get decent power.
The shunt as KT88 triode is not limited by Va (Max) which is 800, but V G2 (max) which is 600V. Also G2 diss might be slightly close to the limit.
First I started looking via DIY Audio for higher-voltage or higher-power pentodes/tetrodes. Almost got there, but nothing quite had all the numbers.
Eventually, from advice, realised there's no need to run the KT88 as triode as Broskie/Bench did, it can be set up as tetrode with G2 at a lower voltage.
It will probably be useful to buy a KT120 though, to have more headroom on dissipation.
Aside from that, I need to get a bigger, badder mains transformer, some higher-rated PP/oil caps, and consider the feedback cascode ( 6N24p) which needs to be something with more voltage tolerance, if I'm running at 700V+ .
Anyway, it now looks fairly do-able to have a shunt-regulated GM70 amp :bounce:
"Once you find out ... the Circumstances ; then you can go out"
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IslandPink
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#215 Re: Future plan for 300B XLS amp

Post by IslandPink »

Just did a quick check on the 300B breadboard, after some inconsistency while finishing the OB speakers, and some odd behaviour by the VR tubes the other day.
Checked continuity of earth from central star-earth to vol pot and input connectors.
All is well on R channel.
Open circuit on L channel :confused3:
Good.. in a way ... improvements to come !
"Once you find out ... the Circumstances ; then you can go out"
nicoch
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#216 Re: Future plan for 300B XLS amp

Post by nicoch »

drive stage news ? el86 C3m 4p1L....
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IslandPink
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#217 Re: Future plan for 300B XLS amp

Post by IslandPink »

Been stable for a while now - 4P1L as pentode , approx. 7.5k anode load, G2 set by OA2 from cathode at 150V . No by-pass cap on cathode resistor.
This driver stage suits me in terms of distortion spectrum and was excellent also driving GK-71 at moderate B+.
"Once you find out ... the Circumstances ; then you can go out"
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