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EL 34 in ST-120

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GreggW

Posts : 57
Join date : 2015-07-15

EL 34 in ST-120

Post by GreggW on Fri Jun 10, 2016 2:28 pm

With a 5V3a, I get a 443 plate voltage. Could I run EL 34s/6CA7s with this plate voltage? Just curious.....
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Tubes4ever

Posts : 71
Join date : 2015-07-14
Location : Star, Idaho

Re: EL 34 in ST-120

Post by Tubes4ever on Sat Jun 11, 2016 12:39 am

Gregg,

Yes you can. Using a 5AR4 on my ST70, I get 420V. Your voltage isn't much more. Just keep the bias down at about .400.

Tim

eickmewg

Posts : 60
Join date : 2014-08-29

Re: EL 34 in ST-120

Post by eickmewg on Sat Jun 11, 2016 6:24 am

Awhile back, I asked Bob if I could use some 6CA7 tubes in my ST-120. He recommended against it, but at the time there was no discussion of rectifier tube choice. Perhaps he will weigh in on this question.
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Bob Latino
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Location : Massachusetts

Re: EL 34 in ST-120

Post by Bob Latino on Sat Jun 11, 2016 8:41 am

If you can get the B+voltage on the ST-120 down below 450 VDC, then you could use EL34's in the ST-120. Try an alternate rectifier tube like a 5U4, 5V3 which will bring the B+ down. You could also consider running the amps off a variac set to 115 VAC or so. This will bring the B+down even with a 5AR4 rectifier.

Consider also that the "range" of the bias system on the ST-120 is set presuming you will be using 6550, KT88 or KT120 output tubes. If you place EL34 output tubes in the amp, the tubes will bias probably way over to one side of the bias pots range and it is possible that one or more tubes may not bias properly at all.

I have been asked this question > "Can I use EL34 type output tubes in the ST-120 amps ?" before. I think much of thought behind this questions comes from the fact that EL34 output tubes are not that expensive and the user is trying to cut costs by using a less expensive output tube in the amp. KT88/6550/KT120 output tubes are $35 - $50 EACH. EL34's can be had for maybe half the price. In the ST-120 you should be using KT88/6550/KT120 output tubes. After 3000 - 4000 hours, they should be replaced. If you use them well beyond their expected life span, they may not bias properly and/or you could have a tube fail in a manner that may take out other components in the amp.

Bob
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GreggW

Posts : 57
Join date : 2015-07-15

Re: EL 34 in ST-120

Post by GreggW on Sat Jun 11, 2016 1:24 pm

Looks like the best bet is to keep using the intended tubes. I forgot about the bias issue. A new driver board and tubes takes away any cost advantage. I wonder if there's much of a "sound" difference between a KT88 and EL34/6CA7 at lower power levels. As usual, great info from all.
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corndog71

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Join date : 2013-03-19
Location : It can get windy here

Re: EL 34 in ST-120

Post by corndog71 on Sat Jun 11, 2016 1:52 pm

GreggW wrote:Looks like the best bet is to keep using the intended tubes. I forgot about the bias issue. A new driver board and tubes takes away any cost advantage. I wonder if there's much of a "sound" difference between a KT88 and EL34/6CA7 at lower power levels. As usual, great info from all.

Having owned both an ST70 and ST120 all I can say is that they both can sound fantastic. Great. Now I want to build another one!
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tubes4hifi
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Re: EL 34 in ST-120

Post by tubes4hifi on Sat Jun 11, 2016 2:43 pm

I'm not here to recommend tubes, but just an opinion of the tubes I've heard. For the ST120, stick with KT88s or KT90s
For the ST70, I do prefer the 6CA7 tube to an EL34, which is why I've got them on my website
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sKiZo

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Join date : 2013-04-01
Location : Michigan USA

Re: EL 34 in ST-120

Post by sKiZo on Sat Jun 11, 2016 3:36 pm

Isn't this the sort of thing the "mystery resistor" (R39) on the VTA board is for? Change that value, and you swing the bias range if I remember right. Default is a bare wire jumper ...
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tubes4hifi
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Re: EL 34 in ST-120

Post by tubes4hifi on Sat Jun 11, 2016 5:26 pm

R39 is part of the bias voltage divider, but never use a jumper, use a 1K minimum, othewise your bias signal will have AC ripple on it.
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sKiZo

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Join date : 2013-04-01
Location : Michigan USA

Re: EL 34 in ST-120

Post by sKiZo on Sun Jun 12, 2016 12:42 am

Something new? The instructions with my ST120 said to use a wire jumper ...



For once I followed the directions, and now you're telling me I'm wrong? clown
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tubes4hifi
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Re: EL 34 in ST-120

Post by tubes4hifi on Sun Jun 12, 2016 12:30 pm

sKiZo,
you're not wrong, but maybe the instructions weren't up to date . . . an RC filter doesn't work as well with a J(jumper)C(capacitor)
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Bob Latino
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Re: EL 34 in ST-120

Post by Bob Latino on Sun Jun 12, 2016 1:10 pm

sKiZo is not wrong .. At the time I was using a jumper for R39 .. Roy cautioned me about the possibility of AC ripple on the bias line but I measure next to nothing with respect to AC ripple with a jumper in R39 OR a 1000 ohm resistor in R39. Adding a 1000 ohm resistor to R39 does not really alter the bias "range" that much and may be a better choice for future VTA ST-120's.

Bob

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