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 Post subject: Cathode current for 5881 tubes
PostPosted: Oct Tue 22, 2019 9:38 pm 
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Location: Aurora, CO 80013
I'm working on a 1960 Knight Basic 60 watt (30 per channel) stereo tube amp. It's supposed to use a quad of EL37 tubes, but those are gone and are quite pricey to replace.
I've got a quad of original Tung Sol 5881s. When using the cathode current jacks to measure current (following the manual's instructions), I get 52ma for V3, 60ma for V4, 56ma for V6, and 60ma for V7. That's with 110vac input. I tried tweaking the Balance pots and this was the closest balance I could get. The unit has been recapped. What's a safe level for 5881 cathode current? Would like to know before I raise the input voltage to 117vac. BTW, no red plating anywhere.
thanks, Dan B


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 Post subject: Re: Cathode current for 5881 tubes
PostPosted: Oct Wed 23, 2019 2:32 am 
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Swap V3 and V7 and they'll be closer (matched pairs have less distortion and better low end). At 400V, this would be up to 24W, higher than the 23W rating of the 5881 - I'd prefer to see 18W or so. The 5881 really needs more negative bias than the EL37 - may be necessary to change a resistor or two. Is this the KB-85? I have a schematic for that one - 260 Ohm cathode resistors should be increased for 5881s.

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 Post subject: Re: Cathode current for 5881 tubes
PostPosted: Oct Wed 23, 2019 3:23 am 
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Tom Bavis wrote:
Swap V3 and V7 and they'll be closer (matched pairs have less distortion and better low end). At 400V, this would be up to 24W, higher than the 23W rating of the 5881 - I'd prefer to see 18W or so. The 5881 really needs more negative bias than the EL37 - may be necessary to change a resistor or two. Is this the KB-85? I have a schematic for that one - 260 Ohm cathode resistors should be increased for 5881s.

Yes, this the same as KB-85. I found a few more 5881 tubes and tried to make better matches, although mutual conductance isn't that helpful for matching. I have PDF of the assembly manual and owners manual. If I increased R44 and R19 from 260 ohm/10 watt, what would be better? thanks

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 Post subject: Re: Cathode current for 5881 tubes
PostPosted: Oct Wed 23, 2019 2:43 pm 
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Location: Rochester NY USA
Looking at the curves, I'd say 360 Ohms would give about 48 mA at 450V, but if you have to order resistors, why not get 330 and 390 as well.

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 Post subject: Re: Cathode current for 5881 tubes
PostPosted: Oct Wed 23, 2019 7:31 pm 
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Increased the resistors to 360 ohms each. I now get 51.3ma on all 4 of the 5881s.

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 Post subject: Re: Cathode current for 5881 tubes
PostPosted: Nov Sat 02, 2019 4:57 am 
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Location: Dos Rios, CA. region
Nice Knight amplifier!
I have a HH Scott 220-A amp that originally came with EL37 tubes I regretfully sold the pair of, some years ago.
From what I've read the 6L6G is a closer replacement for the EL37 than the 5881.
That's what I have (though originally specified) in the 220-A's nowadays...
You might try those?, or 6L6GA perhaps


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 Post subject: Re: Cathode current for 5881 tubes
PostPosted: Nov Sat 02, 2019 2:28 pm 
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Location: Aurora, CO 80013
So far the 5881 tubes are working ok. I do have a quad of RCA 6l6GC and a quad of EL34 I could try. 6L6GA are lower rated than 6L6GB/5881. This amp would fry them. I need to measure plate voltage on the outputs with 120vac input.

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 Post subject: Re: Cathode current for 5881 tubes
PostPosted: Nov Sat 02, 2019 5:54 pm 
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Sounds good Dan,
Did you ever find the Allied manual for this?, I'm curious if any 6L6 subs are listed for the EL37.
It's quite likely with those huge potted transformers the tubes run really cool,
definitely a good idea to measure plate voltage at 120 to see what 6L6 options are possible.
The 5881's will probably last longer in actual use, depending on amount of prior use of course, than other 6L6 types
Kent


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 Post subject: Re: Cathode current for 5881 tubes
PostPosted: Nov Sat 02, 2019 6:40 pm 
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Location: Aurora, CO 80013
[quote="ampstamp88"]Sounds good Dan,
Did you ever find the Allied manual for this?, I'm curious if any 6L6 subs are listed for the EL37.
Yes, there are several manuals available for free download online. The manuals say nothing about subs for EL37, although there are lots of hits for EL37 subs.
The plates are supposed to have 490vdc and the screen grids 495vdc!

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