Newby first build amp 832.

[pre65]

Originally Posted by kuroguy

Not that I have lots of experience here, but maybe a few pointers....
a neatly wired amp is a quiet amp.

Hi-thanks for those pointers.

Can i add that sometimes breadboard amps can sound fine even with a "rats nest" of wires with no hum whatsoever and yet when "put in a box" can sound worse !


Philip

[pre65]

Reading up on the "simple EL84" amp that Andrew I favours i decided to try a mod based on part of the extensive article.

What i decided to try was doing away with the 12AT7/ECC81 valve and substituting a phase splitter transformer.Reason behind this was i thought the preamp gain would be enough with a Pre3.

Now i was advised to try removing the 12AT7 and connecting each 832A grid G1 to signal and ground respectively. My brain could not get round the theory behind this but Andrew I assured me it would work so i tried it !!

Now playing in my second system with valve pre (shunted) and MF MC-2 speakers,and it works !!!!!!!! Volume is a bit low with records but acceptable with CDs,after tea i will try this lash up in the main system with the Pre 3 and more sensitive VoFos,so will report back.

[pre65]

The 12AT7 (ECC81) has now been put back,as the gain was not quite enough, but an ECC82 may be tried in the near future.

As i had some Hammond 156C chokes left over after the ECL82 amp was dismantled to make way for KT66 SET i decided to try them as anode chokes.

They are 150H @ 8ma,and following advice from Nick G were inserted. Just meant repositioning a couple of the existing resistors/caps and drilling 4 holes in the MDF base. So simple really !!

There is a slight improvement,more noticeable on particular tracks,and as no components were purchased a cheap mod.Perhaps i will try a few different interconnects now as suggested on another thread.

[pre65]

Some while ago I ordered four 0.22uf Soniqs SAX capacitors.

Feeling the urge to get "soldering" today I have used them to replace the four "orange drop" caps in the 832a amp.

One of the orange drops had a dry joint to the 12at7 valve base which was remedied and the initial impression on powering up was good.

On certain tracks i can hear tiny details that i was not aware of before ! Perhaps it will improve more as they burn in ?

[pre65]

Some time ago Shane sent me some QQV-07/50 valves for me to try in the 832a,I had offered to see if they would work in that circuit.They did,and had a bit more volume than 832a.

As Shane has now found some of his to be not usable I am sending him back my two and have got some CV2799 (QQV-03/20a) valves to try instead.These were £10 for four,so worth a try ! Bit smaller than 832a in size,but not in sound quantity/quality.

I have them in the amp now,and the initial impression is good. Information on the QQV range of dual transmitter valves is a bit sparse so perhaps that's why they were cheap ?

http://cgi.ebay.co.uk/4pcs-MULLARD-Q...3A1|240%3A1318

One thing, the QQV valve pins are only just long enough to fit septar sockets,and B7a are the right/proper ones to use.

[pre65]

After listening for a few tracks,it seems the new QQV-03/20a were a bit "hard",and i wondered if they were being "over driven" ?

Andrew I had sent me a couple of ECC82 to try, as they have lower gain than the 12AT7/ECC81 and putting them in has lost the "hardness", although the volume control needs a tad more use.

I will try them in the main system later and see how they perform with the VoFos.

[Richard]

Hi Philip,

Sorry, only read the last post in this thread so take this as useful or not try 6072 which has a gain in between 82 and 81, same pinout and is designed for this job. I tried all this series, in gain order, 82, 6072, 81, 5751, 83, on the front end of my amp. They are all the same pinout and the 81 I found is, exactly as Andy Grove once described it I think, chromium plated sound, almost unreal in it's brilliance! 6072 was best by a fair margin in my case but may have been optimised for the stage.

Rich

[pre65]

Thanks Richard-the 12AT7/Ecc81 seems to suit the Russian GU-32 (832a) valve quite well,but not these QQV 03/20a !

The 6072 would be worth trying,but they seem in short supply and not cheap !!

On another forum it was suggested to try 6CG7, but that means altering the heater wiring. I have two 6CG7 on the 300b amp so i could try it without expending any beer vouchers .

[Shane]

Originally Posted by pre65

As Shane has now found some of his to be not usable I am sending him back my two

Thanks Phil. Can't wait to get them back now, 'cos I'm finally in a position where I can start playing with dangerous toys again.

My original intention was to build the same amp as Phil's, but I have a spare pair of C3Gs and a couple of OA2s, so I'm going to have a go at building the front end of DTB's C3G/2A3 concoction, feeding the QQV output stage with one grid grounded to avoid the need for a phase splitter stage in between. I'm not clever enough to know if it'll work or not, but I am stupid enough to suck it and see. If all else fails I can always revert to the original design. If it does work I might then try an output stage in the same configuration but using the quad of 6550s I have knocking around, and use the QQVs to build something to drive my Stax SR3s.

I've also got to rebuild Fakedeck around the Dual 701 motor I seem to have acquired. Don't think there's going to be much time for boating this summer...


Anyone want to buy a Plymouth Pilot 16? (Ducks whilst family rains blows on head).

[pre65]

Thinking about your plan Shane, i have a C3g/150c4 front end on my KT66 set amp (soon to be PT15 !!) and perhaps I could "clip lead" that to the 832a amp output stage ?

Rubs chin and smiles !

Hmm,must think about that.