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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

6DJ8 alternates?

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whitelabrat

whitelabrat

Member
Joined 2006
Hi all! This is a bit off topic with regards to audio, but tube related.

I keep hearing from folks that it's good to have an oscilloscope for DIY adventures. I found two old boat anchors dirt cheap. A EICO 460 and a Tektronics 531. Probably nothing in comparison to more modern scopes, but I've got a better chance of fixing them myself. The EICO is real simple and it seems well maintained. The Tek on the other hand has some real tired tubes in it. Nevermind all the selenium rectifiers, that I'll replace hopefully sooner than later. My real dilemma are all the 6DJ8 (ECC88) tubes. Geez, this thing is packed with them! At least 10 I guess.

Any suitable alternates out there? 6N23P's? 6N1P's won't do. Attach a chain and toss the scope overboard?

The wiring is all point-to-point so it may be possible to use things with different pinouts.

I could always turn it into an amplifier right? :D
 
A

Allen Wright

R.I.P.
Joined 2001
vacuumstate.com
In my experience, fixing up an old tube scope is a nightmare, and only for the genuinely crazy or those who want to do it as a labour of love.

I suggest pulling all those possibly Telefunken or Bugle Boy 6DJ8's, selling them on ebay for $50 each to freaks in Japan, and buying a used but much more modern solid state scope.

A couple of months ago I got a HP182C scope in PERFECT condition for circa $180 of ebay.

IMO, test equipment is for use in making your audio work, not for killing your time. Unless you have a huge amount of time and want a HUGE challenge!

Regards, Allen
 
Geek

Geek

Member
Joined 2004
whitelabrat said:
The Tek on the other hand has some real tired tubes in it. Nevermind all the selenium rectifiers, that I'll replace hopefully sooner than later. My real dilemma are all the 6DJ8 (ECC88) tubes. Geez, this thing is packed with them! At least 10 I guess.
Click to expand...

Is that one of those great scopes that had 100MHz BW when everyone else was still struggling to break 2MHz?
 
T

thevoice

Member
Joined 2005
I have a HP scope with what sounds like a similar tube complement, about 10 Telefunken ECC88's. I managed to pick up about 20 replacement tubes on Ebay for $2.50 each. They are made by god-knows-who, branded Lewis and Kaufman and I have no idea how they would be for audio, they should be fine for a scope though.

Keep your eyes open, there are deals to be had.
 
whitelabrat

whitelabrat

Member
Joined 2006
Oh dang. This one uses 6BQ7's instead of 6DJ8's. Excellent! 6BQ7's are cheap!

This scope is a 15Mhz I believe. That is pretty good for a late 1950's scope I guess. It also has a very cool blue phosphor! It's in very good condition too. I just can't junk it!

Any questions about the sanity of my sanity in trying to do this is completely valid! :tilt:

I did pull about four good Telefunken 12AT7's. A RCA 12AT7 blackplate blew a triode to my disappointment.
 
whitelabrat

whitelabrat

Member
Joined 2006
It does get nice and warm. I measured almost 300watts being drawn. I figure I'd replace the bad tubes, use it as a learning tool and then salvage the enormous power transformer when I'm done. I've already got quite a few tubes for it (12BY7's, 6AU5, 12AX7, 12AU7, 12AT7, 6AU6, etc), so why not have some fun with them?

Anyone try 6BQ7's for audio application before? I suspect they aren't too far off from the ECC88's if they're being used as an alternate in the scope.
 
richwalters

richwalters

Member
Joined 2003
tubelab.com said:
I had a 531 about 25 years ago. Makes an excellent room heater!
Click to expand...


My early days in defense electronics I often used an HP spectrum analyser, it came in several stackable parts (think it was 855B ?) was so large and the Watkins-Johnson backward wave oscillator had a magnet so powerful one couldn't get a screw driver off it.
packed with tubes right through, any of this "real" electronic stuff still around ?

rich
 
J

Jim McShane

Member
Joined 2006
whitelabrat said:
It does get nice and warm. I measured almost 300watts being drawn. I figure I'd replace the bad tubes, use it as a learning tool and then salvage the enormous power transformer when I'm done. I've already got quite a few tubes for it (12BY7's, 6AU5, 12AX7, 12AU7, 12AT7, 6AU6, etc), so why not have some fun with them?

Anyone try 6BQ7's for audio application before? I suspect they aren't too far off from the ECC88's if they're being used as an alternate in the scope.
Click to expand...

I've got a few customers who use 6BQ7/6BQ7As in their audio gear. They seem to like them a lot, but that's about all I know.
 
richwalters

richwalters

Member
Joined 2003
I still find it amazing at this time some 40 yrs after ECC88 types lost out to the silicon and germanium, that the world is still full of surplus and still being manuf'ed.
Now that another thread has wetted my appetite to the EL156, the KT88, 6550 user market has been boring slow in revamping this monster pentode. How on earth did the KT90 ever get barged in ?
I know the EL156 socket is different but that is easily solved.
Shugang appears to be the only manufacturer of the EL156 correct ?

rich
 
Miles Prower

Miles Prower

Member
Joined 2005
dolphin-hsl.com
whitelabrat said:
Anyone try 6BQ7's for audio application before? I suspect they aren't too far off from the ECC88's if they're being used as an alternate in the scope.
Click to expand...

Actually, yes. Le Renard Main Schemo

6BQ7As have right around half the g(m) that the 6DJ8 has (6.4mA/V as opposed to 12.5mA/V). Still, the 6BQ7As have good sized cathodes that glow nicely. This type was never intended for audio use, and the linearity isn't outstanding. You'd probably not want to use this for any conventional audio circuit, as there are better VTs available for that. There seems to be something funkey about the characteristic, like maybe a variable-u characteristic that's not documented. The trick to getting good linearity is to keep the Vpk up.

However, for the cascoded LTP, they sound great, have more than enough gain to eliminate another gain stage and still provide enough gain margin for gNFB with reasonable input sensitivity. Since this is a type that never had audio pretensions, they do tend to be microphonic. You'll have to either shock mount the sockets, or otherwise protect the amp from vibrations. (Using these for a gee-tah amp inside a cab is definitely out.) Other possibilities are a straight LTP splitter, SRPP or Mu-stage (though I haven't tried that yet).

Since the 6BQ7A is a VHF type, if you don't feed it RF it'll make its own. That means taking precautions to avoid instability. Certainly grid stoppers, but also use good RF construction practices: all leads as short as possible, cross socket baffle shields, baffle shields between stages.
 
richwalters

richwalters

Member
Joined 2003
Miles Prower said:


Since the 6BQ7A is a VHF type, if you don't feed it RF it'll make its own. That means taking precautions to avoid instability. Certainly grid stoppers, but also use good RF construction practices: all leads as short as possible, cross socket baffle shields, baffle shields between stages.
Click to expand...

Miles;
Good point. Same precautions with ECC88 types. Those benchers who can work with RF and Video tubes in audio appls are in a world of their own and the going is far cheaper. Although the 12BY7 video group are my favourite user tubes, there are hoardes or others also cheap and provide superb performance with sensible layouts. The surplus 12HG7 (gm/v !!) a remarkable gonner, and remarkably consistent and nice with fat cathodes.

In some ways not far akin to mosfet performance but more robust to abuse.

All this makes the ECC82/3 group look a bit deaf for dealing with upper digital audio bandwidths unless considerable feedback is used to stretch the response.

richj
 
coresta

coresta

Member
Joined 2005
Hi , i'm only working with TEKs in my lab . In Summer , a 5000 (silicon) and in winter : a 500 :D THos are hard to beat : blue or green screen, 30 or 90MHz BW, and even a 551 twin gun :clown:
 

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whitelabrat

whitelabrat

Member
Joined 2006
I'm just about finished on my interpretation of the Bruce Bender variation Morgan Jones headphone amp. It's design is for 6N1P tubes. I've done a completely different power supply though where I use a cheaper 500vct power supply and a choke along with some salvaged caps. During assembly I realized I was missing some resistors so I paralleled some to get the right value, but I was stuck with a 50k resistor where I needed a 43k. I used it anyway and ended up with 100v on the input tube where I needed about 170v. So here's where I make my point.

I dropped in a 6bq7 in place of the 6N1P tube on the input stage and it worked like a champ! Sounds good. I'm afraid my TEK 531 will never get it's tubes back :)
 
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