Going to do the 2203 6550-EL-34 conversion

[Rampage]

Well, I've been busy lately and have put off a retube on my 2203, but I think it's time to switch from 6550s to EL-34s. I've been looking around, and I am just going to go the simple route and "piggyback" another 47K resistor on the existing 47K bias range resistor. Quick question though: Which type of resistor have you guys that have done this used? 1/2 watt metal film 47K ohm resistor?

Also, has anyone used the Amp Head Dual BiasTester with the toggle for plate voltage and bias current? I really like the idea of being able to get the plate voltage without having to check the reading from pin 3 by hand. It also seems like a convenient way to quickly measure the bias. http://www.amp-head.com/product_info.ph ... 70b09f315f

 

[Riddick]

You will have to change the bias splitters too. The originals are either 150K or 82K they need to be changed to 220k.

 

[Rampage]

You will have to change the bias splitters too. The originals are either 150K or 82K they need to be changed to 220k.

I read that you can leave the splitters and supply resistor at the stock values because the piggybacked resistor on R28 provides enough negative voltage to get into the right bias range for EL-34s.

The two options I saw are as follows:

A. Piggyback a 47K resistor on the bias range resistor and bias for EL-34s.

B. Replace the bias range resistor with a 56K, replace the splitters with 220K resistors and replace the supply resistor with a 220K as well. You can also screw around with the NFB wire, but I've heard that isn't really necessary.

I would like to stick with the piggybacked resistor for the time being as it would be a lot faster, plus, I wouldn't have to pull the entire board and give myself an extra opportunity to mess up.

 

[Riddick]

Give it a try and see if it is in range to bias, sometimes you have to play around with that resistor to get the bias within range. I always just change out all of them and be done with it myself.

 

[MississippiMetal]

I would also install some larger 1 ohm resistors between pins 1 and 8 in place of the wire, or between pin 8 and the chassis tab(I prefer to remove those sources of ground reference myself, but that's another story). That way you can measure the voltage across it, subtract 5mV (for screen current) and have your plate current. A little safer than measuring the current through the OT primary.

 

[joepete77]

every old unmodded marshall i have played or owned with 6550 i didnt like nearly as much as with el34. The 6550 seemed like they had really high headroom and wouldnt brake up nearly as quickly as the el34. I recomend the switch as its fairly easy and u can always go back if u dont like it.