Caveat on using 6BQ5's in modern EL84 amps!

[Steve A.]

I picked up a quintet of NOS 6BQ5's recently but wanted to make sure that they would work properly in a few amps I have around. I seem to remember some warning about not using any of the IC or NC pins as jumpers. Well, here it is (short version: do not use pins 1, 6 or 8 for jumpers)...

From the Ampage Archives:

6/6/2000 2:43 pm
MBSetzer
Re: Wiring up EL84's
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Hi All:

IMHO, pin 1 is not a good idea . . . [Editors note: previous post had asked about using pin 1 as a jumper]

Here is some old data I gathered:

Hello fellow tubeheads:

In the GE book, 6BQ5 & 7189 share the same basing pinout: 9CV;

2 is grid
3 is internally jumped to both cathode & suppressor
4 & 5 are heaters (6.3Volts)
7 is plate
9 is screen
1, 6, & 8 show *IC* which is taken to stand for *Internally Connected*, which I understand means that the pin at least goes through the glass envelope and its metal is exposed to the high vacuum inside. Whether or not an *IC* pin actually connects to an internal electrode, and which one, does not seem to be universally required unless the basing diagram shows a connection to a particular electrode, 9CV does not, and 1, 6, & 8 show no guaranteed connection whatsoever.

On my GE 6BQ5 tubes which would be assumed to correspond to their own handbook, everything is as I listed above, and also pins 1 & 2 are jumped internally. Pins 6 & 8 are there going through the glass, but have no electrical connection to each other or anything else inside. These GE tubes have 6BQ5 USA etched on the glass with the typical dot pattern below, in white writing there was added GE 6BQ5/EL84. The box also says 6BQ5/EL84.

My RCA 6BQ5's are the same way, also RCA 7189, as well as recent Ei 6BQ5 from Yugoslavia. The Sovtek EL84 & EL84M do not have any connection to pin 1, so they actually follow the basing diagram more closely than the others. I do not have any true NOS European EL84's to compare, but it is considered the exact substitute for the traditional US 6BQ5. On my RCA's the 6BQ5 is applied within their typical octagon with USA above it, additionally painted on the glass is 6BQ5/EL84, the box has 6BQ5/EL84 as well. My 7189 tube is identified within the octagon with USA above it, indicating RCA manufacture, but this NOS one was relabeled and marketed by Raytheon with an apparently original box, except the box shows 7189A, this tube does not have pins 6 & 9 internally jumped though, see information below about actual 7189A. The Ei is only marked Yugoslavia 6BQ5 on the tube, the box has EL84/6BQ5. The Sovteks are visibly different from each other, supposedly made in different plants, the standard tube has EL84/6BQ5 on the glass, the box only EL84. The Sovtek EL84M is painted only that way on the glass with Made in Russia shown in English, there are additional Russian designations including what is likely their tube number, 6Pi14Pi-EP, with Pi representing the single cyrillic character equivalent to the greek letter *Pi*. This tube appears to have a date code of 9108, from 1991. Both Sovteks have the thin shiny pins these are known for.

In the GE book, the 7189A has a different basing all its own: 9LE;

Same as above for the most part:

2 is grid
3 is internally jumped to both cathode & suppressor
4 & 5 are heaters (6.3Volts)
7 is plate
9 is screen

On this one the difference is it clearly shows an internal jumper between pins 1 & 2, but also it has pins 6 & 9 shown jumped internally as well.

My only 7189A appears to be falsely labeled, it was marketed by Westinghouse in their notorious later violet box, on the tube is printed 7189A Westinghouse Made in Germany. It appear identical with the Sovtek EL84 except the pins are not the thin shiny ones like Sovteks have currently, but look more like traditional US NOS pins. It is electrically identical to the Sovtek EL84, and there is no implementation of the 7189A pinout specification.

What this means is that if you use only pins 2, 3, 4, 5, 7, & 9 in an amp design, it will be pin-compatible with all of the above tubes. Of course it's up to each operator how hot you want to stress yours and with how high a voltage

part2

If an amp calls for 7189A and doesn't work with 7189, 6BQ5, or EL84, I would expect that the designers were using pin 6 instead of pin 9 for their screen connection, this would be easy to correct, but maybe it was a stressful circuit and not intended to use commercial tubes. If 7189A doesn't work in a 6BQ5 or 7189 circuit, there could be the possibility that unused pins (1), 6, & 8 were utilized during assembly merely as utility soldering terminals (pin 1 would be really dumb apparently) for connections not intended to be made to the tube to begin with. This can easily be modified for broader substitution possibilities. There is also some possibility at one time that US 6BQ5's did not have an internal jumper between pins 1 & 2 either, but my specimens all are jumped.

Additionally, there is available a Svetlana SV83 not claimed to be usable in an EL84 circuit, but according to their literature *EL84 can be used in an SV83 CIRCUIT without modification.* Only a jumper on the socket. I don't have one of these tubes, but from their technical literature I would suggest the above (2, 3, 4, 5, 7, & 9) wiring to the circuit, with an external jumper from the cathode pin 3 to their suppressor pin 6 would make it socket compatible with all tubes mentioned except a true 7189A. Their suppressor appears to be internally jumped to pin 1, another good reason to always avoid connecting pin 1 to anything when using any of these semi-compatible small power tubes. Lifting the connection from pin 6 would allow 7189A use when desired. The screen of SV83 has a lower voltage and wattage rating however, so that must be taken into consideration, otherwise the tube is similar and is reported to sound quite good.

Hope this helps,
MBSetzer
*Progress is a continuing effort to make things as good as they used to be* - - - Sam Ewing

[Steve A.]

I was looking at my Crate V18-112 and they are using pins 1, 6 and 8 as jumpers.

Just wondering how feasible it would be to cut the traces around the tube pin on these pcboards- is there a tool like a hole saw to cut out a neat circle around the copper pad? Or maybe even cut the terminals going from the tube socket to the board...

If anyone has dealt with this before I'm searching for ideas here.

Thanks

Steve Ahola

P.S. Be sure to inspect your 6BQ5's both visually and with a meter to make sure that pins 1, 6 & 8 aren't connected to anything. I would not rely on the spec sheets for this.

[Diablo]

How about you just remove the pins that aren't needed on your tube with a dremel cut off wheel?

[Steve A.]

How about you just remove the pins that aren't needed on your tube with a dremel cut off wheel?

Just playing the Devil's Advocate, Diablo?

Well, I actually did think of that. It would probably work better with an octal tube with the bigger pins and wider spacing. Since the problem is really more with the amp than the tubes I think it would be better to fix the amp so that it would work properly with all 6BQ5 and EL84's. And if I ever try to sell these NOS tubes (as used Old Stock, of course) there would be less questions to answer. "Oh, THAT? I didn't particularly care for those pins so I just cut them off..."

But speaking of cutting pins on EL84's I ran across a half-power mod for the Classic 30 that involved cutting off the 2, 7 and 9 pins (since the C30 runs the power tube filaments in series if you pull two tubes the other tubes won't work). And of course you would do that with junky Sovtek EL84's that you pulled out of a $99 amp...