Decca DMI-73 and EL-84 weirdness - diyAudio
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Old 27th September 2008, 02:46 AM   #1
uliami is offline uliami  United States
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Default Decca DMI-73 and EL-84 weirdness

I just got a DMI-73 and wanted to try some new tubes in it. It has Crestmark 6BQ5s in it, I put new Electro-Harmonix EL-84s in it and lost the tremalo and had half the power, It looks like 1 power tube powers the tremalo, because I put 1 6QB5 in the spot next to the tremalo tube and it returned, sounded bad tho, reinstalled the other crestmark and all was normal. Tried another set of EL-84s and the same thing. I know their the same tube but its what happens. Any thoughts on this?
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Old 27th September 2008, 02:55 AM   #2
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I dunno. Check your old tubes to see if they are internally connected between pins 1 and 2?
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Old 28th September 2008, 01:26 AM   #3
uliami is offline uliami  United States
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By golly they are internaly connected, so what strange tube is this, and what would I have to do to use a modern EL-84, put a jumper on the sockit of both tubes?
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Old 28th September 2008, 04:43 AM   #4
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6BQ5 aren't supposed to be connected, but sometimes they are. 7189A are supposed to be connected, but I haven't had the luxury of owning any to draw any conclusions. Look up the datasheets using something like http://tdsl.duncanamps.com/

Check the circuit of your amplifier carefully. Look for the input signal to the 6BQ5 tubes. It needs to get to the grid of the tube. The grid also needs to be connected to ground via a ~300K ohm resistor (known as the "grid leak"). If you leave the grid floating, it will accumulate a charge within several minutes. Then the whole tube will conduct like mad and begin to glow red.

If something is wired to pin 1 and nothing is wired to pin 2, move that connection to pin 2 and try it again. Your jumper idea might work too, but I'd rather just make sure the grid connection goes to pin 2 (where it belongs).
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Old 28th September 2008, 03:38 PM   #5
kevinkr is offline kevinkr  United States
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That might also be an indication that the amplifier really wants 7189(A) or equivalent. I have a Japanese made amplifier (Realistic Stereodyne 40) that wired pins 1 & 2 together and had Japanese 6BQ5 in it, installing new JJ or NOS GE types resulted in a "Super Nova" experience.. Russian 6P14P-EV solved that issue.

The B+ in that Realistic amplifier by design according to the schematic is a smidge below 400V with modern line voltage it is slightly over. The output stages are cathode biased at 100mA for the pair! (Deliberately) And the original tubes lasted a very long time, whereas anything I have stuck in it since has been quite short lived.

The Japanese 6BQ5 appears to be much more rugged than US or recent European versions as this amplifier and several others have proved. My dad had one of these amplifiers (not the one I have) and I did retube it at some point with Mullards (back in the day.. LOL) which seemed to hold up ok in that particular amplifier.

YMMV, measure your plate voltage and if over 350V I'd replace them with 6P14P-EV to play it safe and jumper pins 1 & 2 together. (You can get these very inexpensively on eBay and they are excellent.)
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Old 28th September 2008, 07:16 PM   #6
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B+ at 400 volts, cathode biased, 100 mA per pair? That's some tough punishment. Even if you drop 15 volts in the cathode resistor and another 10 over the OPT's primary, you're still looking at nearly 19 watts dissipation each tube. That's a job for a 6GM5, not a 6BQ5.

I run the 6p14p-ev under similar voltage conditions, but keep it under 65 mA per pair. Excellent tubes, and they're perfectly happy running there. However, I just checked one of mine and there is no internal connection between pins 1 and 2.

Click the image to open in full size.

I've got some old, brown-stained Mullard. They have no silkscreening left on them, and even the etched factory codes are tough to read. However, you can clearly see the internal connection between pins 1 and 2.

Click the image to open in full size.
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Old 28th September 2008, 08:29 PM   #7
pchw is offline pchw  United States
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This reminds me my Fisher X-100-3.
When I bought the amp used a couple years ago, I really didn't know the difference of equivalent tubes such as 7189 and 6BQ5. This one calls for the 7189, but I retubed it with Sovtek 6BQ5 and gave it to my father to use. He really didn't use the amp that much, but the tubes seemed to age very quickly. Then I learned the difference between 7189 and 6BQ5 here :-) Since then, the Russian 6P14P-EV's have assume the duty from the Sovtek's. Comparing the 2 pes, the 6P14P-EV are more open and have more slam in the bass department, and the price is right, too.

BTW, the X-100-3 is running at 390 B+, 360V across the tube (grid 18, cathode 30), and 38ma through each tube, no where near Kevin's Realistic amp :-)
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Old 29th September 2008, 01:27 AM   #8
uliami is offline uliami  United States
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Well I open it up, and they use pin 1 on both tubes and pin 2 has nothing on it. So I guess I could jumper the 2 and use ether the 7189A or the 6P14P-EV.
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Old 29th September 2008, 03:35 AM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by uliami
Well I open it up, and they use pin 1 on both tubes and pin 2 has nothing on it. So I guess I could jumper the 2 and use ether the 7189A or the 6P14P-EV.
Sounds like the right thing to do..

Eventually I will reduce the idle current in the output stage of that amplifier. Interesting that my dad's ran for over 20yrs with just one change of output tubes over that time.. I have the sense that mine will not run more than a year or two with these tubes were I to use it on a regular basis.
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Old 4th May 2011, 01:56 AM   #10
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Default Decca Dmi -73 retube

how did you make out with your retube of the Decca DMI-73 amp. Did you move the wire from pin 1 to pin 2 in order to use the EL-84 tubes?
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