Tube swapping (6550-to-EL34)

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Tube swapping (6550-to-EL34)

Postby ragtweed on Wed Feb 18, 2004 10:24 pm

Any tube guru's know if I can swap out a pair of 6550's in a combo amp and replace them with EL34's? Obviously, the bias and maybe even the bias circuit would have to be tweaked, but is it feasible? Its a Sundown (built by Dennis Kager) from the early eighties. Its pretty overpowering even at moderate settings and I'd like to tame the beast if at all possible. Otherwise it'll probably end up on e-bay!
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Re: Tube swapping (6550-to-EL34)

Postby Tube Tramp on Wed Feb 18, 2004 11:05 pm

of course it is you silly man .what do you think these pilgrims do when they change out their jcm800 spandex amps.? lol just rebias mow and go................if id a known this in 76 i wouldnt of had to damn near throwaway a nice new 50 i bought couldnt stand it with the 6550s had no idea they could be changed by simply dropping el34s in it and a bias ...........tt
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Re: Tube swapping (6550-to-EL34)

Postby ragtweed on Wed Feb 18, 2004 11:54 pm

Thanks TT.
A couple of tube dealers advised me not to even try it. I know the KT88's would work safely, but if anything the RMS output would increase. The EL34's should be able to cope with the plate voltage and I figured since the pinout arrangement is pretty close to a 6550 it might work. I don't know if I need to run a jumper between pin 1 and pin 8 or just plug in the EL34's and set the bias. I'll find out quick when I turn it on I suppose.
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Re: Tube swapping (6550-to-EL34)

Postby goneracin on Sat Feb 21, 2004 12:34 am

Go to Voodoo amps website. They have a step by step list for the conversion. I think its under the FAQ section maybe, or tech section. Its an easy mod, just change a couple resistors, and the NFB wire if desired.
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Re: Tube swapping (6550-to-EL34)

Postby ragtweed on Sat Feb 21, 2004 1:19 am

Thanx goneracin',
I found the site and it sounds like they do some pretty cool amp mods. I couldn't find any specific info pertaining to the power tube conversion although one of their standard mods is an adjustable bias circuit.
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Re: Tube swapping (6550-to-EL34)

Postby HighOrder on Sat Feb 21, 2004 1:36 am

MARSHAL 6550 to EL34

1. Locate the 47k resistor above the bias pot and solder another 47k in parallel.
2. Set the bias pot to the max setting (around 38 Volts). The direction to turn the pot is indicated on the circuit board.
3. Locate the purple feedback wire that is connected to the 4ohm speaker tap (black wire connected to the impedance selector) and solder it onto the 8ohm speaker tap (yellow wire connected to the impedance selector).
4. Install the EL34's and re-bias the amp.

There are some other things that you can do to make the conversion more "complete," but they're really not necessary, IMO. These include swapping out the 150k bias splitting resistors with 220k, and changing the value of the current limiting resistor in the bias circuit (maybe one or two other things, I can't remember off hand).

I converted mine from 6550s and made several other changes, but now it's back just the parallel bias resistor and a 220pF bright cap ... except for one thing. I put a 220k resistor in parallel with the 100k negative feedback resistor (so I have about a 68k NFB resistor). I really like this combination for big yet tight bottom. I got the idea from another board member (thanks Ian). :D
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Re: Tube swapping (6550-to-EL34)

Postby ragtweed on Sat Feb 21, 2004 2:01 am

Thanx High-Order,
I'm modding a Sundown SD1012C combo amp. You did this on a Marshall I presume? Some things are going to be different on my amp, but at least it sounds like it can be done.
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Re: Tube swapping (6550-to-EL34)

Postby HighOrder on Sat Feb 21, 2004 2:32 am

Originally posted by ragtweed:
Thanx High-Order,
I'm modding a Sundown SD1012C combo amp. You did this on a Marshall I presume? Some things are going to be different on my amp, but at least it sounds like it can be done.
Hmmmmm ... yeeessss ... sorry, I didn't notice that we weren't talking Marshall here. The key thing is to get the bias voltage negative enough to control the cathode current. You can look it up, but I think it should be in the -50V to -40V range for 6550s and -40V to -30V for EL34s. So if you're set up for 6550s, you may not be able to bring it up enough (e.g., more positive ... or less negative, if you will) as it is. It might also work though, so you might want to check the range of adjustment with the power tubes removed.

If you've identified the bias circuit, and it's similar to the Marshall circuit, you should be able to do the same thing described above to bring it into range. Normally, it's a basic half-wave rectifier type set-up ... A/C to a diode/current limiting resistor feeding a capacitor that sets the voltage across a series pot and resistor ... something like that. The resistor value that you're changing is that of the resistor in series with the pot. This changes the overall range of adjustment, lowering the max negative voltage when adding a resistor in parallel to the existing value.

Don't use the voltage alone to bias, of course. If you have a bias probe like the Weber Bias Rite (I have the BR-4 ... very nice), monitor the cathode current and keep the plate dissipation around 70 percent of max for the tube type. At least that's the recommended range for most Marshalls. Used your ears to find the sweet spot, but don't push the tubes too hard.
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Re: Tube swapping (6550-to-EL34)

Postby ragtweed on Sat Feb 21, 2004 3:23 am

Thanx again HighOrder,
There might be something wrong with my bias circuit. I checked the negative voltage on pin 5 (w/6550's) and its around minus (-)63v and if I check the bias current with my Bias King its only like 17ma through each tube. I'm going to have to do some homework to set this up right.
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Re: Tube swapping (6550-to-EL34)

Postby HighOrder on Sat Feb 21, 2004 3:34 am

Originally posted by ragtweed:
Thanx again HighOrder,
There might be something wrong with my bias circuit. I checked the negative voltage on pin 5 (w/6550's) and its around minus (-)63v and if I check the bias current with my Bias King its only like 17ma through each tube. I'm going to have to do some homework to set this up right.
Actually, that doesn't sound wrong to me. I'd think -63 would set those 6550s really cold, as indicated by the 17mA. If you can't adjust it to get the tubes to run hotter (say -53V instead of -63V), then you might have a problem.

If there's no bias adjustment pot installed, you might be able to mod the amp to add the pot or work the math to determine what resistor value(s) to put in the circuit to bring things into line. I'm not familiar with this particular type of amp though.
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Re: Tube swapping (6550-to-EL34)

Postby scobie on Fri Jul 25, 2014 2:39 pm

Hello, my name is Rodney and I live in Northern Ireland, I wondered if you could maybe help me? I have had a Sundown Artist 100w combo amp for over 25 years and still gig with it, fantastic amp. Unfortunately the RMS pot needs replaced but I have had no lucjk sourcing one anywhere in the UK. I have been in touch with Dennis Kager, the man who built and designed these amps, but he does not have any. THe original pot was 5 watt but Dennis said that a 10 watt would be better and anything up to 300ohms would do as long as it fits on the circuit board. Would you by any chance know where I would get one of these, sorry to just hit you with this, a stranger out of the blue, I am new to this forum, but I am clutching at straws now and pulling my hair out, HELP!!!!
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Re: Tube swapping (6550-to-EL34)

Postby dtw576 on Tue Aug 05, 2014 9:55 am

scobie wrote:Hello, my name is Rodney and I live in Northern Ireland, I wondered if you could maybe help me? I have had a Sundown Artist 100w combo amp for over 25 years and still gig with it, fantastic amp. Unfortunately the RMS pot needs replaced but I have had no lucjk sourcing one anywhere in the UK. I have been in touch with Dennis Kager, the man who built and designed these amps, but he does not have any. THe original pot was 5 watt but Dennis said that a 10 watt would be better and anything up to 300ohms would do as long as it fits on the circuit board. Would you by any chance know where I would get one of these, sorry to just hit you with this, a stranger out of the blue, I am new to this forum, but I am clutching at straws now and pulling my hair out, HELP!!!!

Don't know that amp but if these guys don't have the component you need, or something that may substitute and work, no one will. They are "the" place in the USA that has most everything electronic.

http://www.mouser.com/
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Re: Tube swapping (6550-to-EL34)

Postby ardencaple on Tue Aug 05, 2014 2:55 pm

scobie wrote:Hello, my name is Rodney and I live in Northern Ireland, I wondered if you could maybe help me? I have had a Sundown Artist 100w combo amp for over 25 years and still gig with it, fantastic amp. Unfortunately the RMS pot needs replaced but I have had no lucjk sourcing one anywhere in the UK. I have been in touch with Dennis Kager, the man who built and designed these amps, but he does not have any. THe original pot was 5 watt but Dennis said that a 10 watt would be better and anything up to 300ohms would do as long as it fits on the circuit board. Would you by any chance know where I would get one of these, sorry to just hit you with this, a stranger out of the blue, I am new to this forum, but I am clutching at straws now and pulling my hair out, HELP!!!!


Actually, Scobie, that is trickier than you might imagine. High power, wirewound potentiometers are very difficult to get hold of in the UK now.
Rapid Electronics have a few, but they are 4W rated. See http://www.rapidonline.com/electronic-components/250r-wirewound-potentiometer-4w-aw-series-50-3983

I could not find a schematic that matched exactly, but this one may be an artist 50? http://www.blueguitar.org/new/schem/sundown/sndn_a50.gif

It does have an RMS pot marked on the schem, so the function should be similar; it is an adjustable bias resistor. OK, so the question is, what output tubes does your amp use? 4 6l6s?

The point is, that if we know what the typical bias voltage is, at the typical bias current, we can then work out what the minimum resistance would be. If you connect a high power resistor of that value in series with the adjustable resistance made from the pot, the power will be shared between the resistor and the pot; this means a lower rating for the pot is OK.

At this point I should confess that the following is theoretical, as I have no experience with 6L6s or 6550s, so I should get a second opinion ... Anyway, the data sheet seems to indicate that for two 6550s in class AB, with cathode biasing, the cathode resistor should be about 140 ohms. For 6L6s, I calculate about 100 ohms. For a 100 watt amp, with four tubes, the resistance will be a half of that.

So it looks like the typical resistance will be set to 50 - 70 ohms. In that case, we could place a 39 ohm, 10 watt resistor in series with the pot, and that should still leave some room for adjustment for hotter / colder biasing.
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Re: Tube swapping (6550-to-EL34)

Postby ardencaple on Tue Aug 05, 2014 6:23 pm

Ah. My Bad.

Actually, the RMS control looks like it adds in a variable amount of cathode bias - on top of the fixed bias. So at maximum power, you set the RMS control so that the extra cathode resistance is 0.

Hmm. I suspect you might be able to get away with a 4W part, certainly at minimum and maximum settings (at maximum, there is no resistance added in, so there will be no power dissipated in the pot; at minimum setting, the current will drop way below it's normal value).
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