Switching Nitro from KT88's back to EL34's... not so cut and dried.

War_in_D

War_in_D

Well-known member
I finally got around to pulling the chassis of the Nitro out of the headshell to see what would be involved in taking this amp back to EL34's from KT88's. A little background first. When I bought this Nitro, it was originally EL34, and I sent it back to Scott to have the "Fully Loaded" mods done (New/Old Switch, Mid Switch and B+ switch). While it was at Scott's shop, I talked to him about converting it to KT88's and decided to go that route. Fast forward, and now I'm thinking that I might want to convert it back to EL34's and I had looked at the thread over at Splawn proboards forum to see how to do this. Looked pretty straight forward and easy to do. Welp, it's not so cut and dried in my case due to the B+ switch being added. The original resistor for the bias adjustment was in position R30, and when I cracked open my Nitro.. this is what I saw.

There is a jumper from the R30 spot on the PCB, going to the B+ swtich top/bottom lugs and there are two resistors between the top/bottom lug that connect to a middle lug. Soooo... Yeah. LOL

I'm pretty much an idiot when it comes to electronics, so this may be a job for the local tech. Especially not knowing what resistor values are there, and what is needed. Someone more well versed in this type of thing might know what they are just by looking at them, but that someone is not me. LOL

IMG_20220421_095428150.jpg


Nitro B+ switch to bias resistor.jpg
 
I'll tell you my take but I've never liked B+ switches because I always like one voltage over the other and never use it.

I don't see how tube type matters regarding the B+ switch aside from bias. My guess is that it is changing the B+ AND the bias voltage proportional to the B+. But there should be a bias range resistor that sets the 'nominal' bias voltage for that tube type and is then further dialed in with bias pot for that set of tubes.

Plane is taking off, have to stop for now.
 
Take bias voltage readings on Standby with B+ in both positions and write them down, maybe post here. You could also pull power tubes and take B+ readings in both switch positions
 
SpiderWars said:
I'll tell you my take but I've never liked B+ switches because I always like one voltage over the other and never use it.

I don't see how tube type matters regarding the B+ switch aside from bias. My guess is that it is changing the B+ AND the bias voltage proportional to the B+. But there should be a bias range resistor that sets the 'nominal' bias voltage for that tube type and is then further dialed in with bias pot for that set of tubes.

Plane is taking off, have to stop for now.
Click to expand...

Yeah, I can see that now regarding the B+ switch and setting bias. I'm sure that a person would be better of to pick one setting and stick with it. Once I saw how that was set up, I figured with the two resistors there must be one for each side of the B+ switch on order to set the bias range for each B+ setting. I just wouldn't know what to change the resistors to for the bias to land within the adjustment range and give it enough room to bias up with the EL34's. I thought about just sticking the el34 is in there and seeing if I could get them to bias with the resistors that are already there but I don't want to jack anything up.

SpiderWars said:
Take bias voltage readings on Standby with B+ in both positions and write them down, maybe post here. You could also pull power tubes and take B+ readings in both switch positions
Click to expand...

I'll do that and see what I can come up with. Thanks.
 
Hey @SpiderWars

I guess I'm dense, but when you say take a reading of the bias voltage, I guess I was thinking plate voltage and plugged the Nitro up into my Weber Bias Rite and went to read plate voltage from that, and of course.. it's reading zero when the amp is on standby. If I wanted to get bias voltage with a multimeter, where would I need to measure to get that? Would it be across the terminals where R30 is?
 
Also, as just an FYI.. Since I've got the Weber hooked up, I decided to see what difference the B+ switch makes in the plate voltage on this Nitro.

Regular volage: 493v
B+ Engaged: 425v

The bias shot up from 52-ish (a bit high, should be closer to 50) @ 493v, to around 74-ish @ 425v! HOLY MOLY! So yeah, I'm seeing where the B+ switch could be an issue if you were to run it for any extended length of time. Talk about putting the coals to the tubes!! LOL

I think while I've got it all out and hooked up, I'm going to throw some EL34's in there and see where they land.
 
Ok, so I think I can make this work. I just need to find two EL34's in the same bias range. Here is my plan, someone tell me if I'm doing something wrong or mistaken in my thinking.

I'm going to run the Nitro at half power, and remove the two outer tubes. Checking the plate voltage with the EL34's in there, at full power I'm getting around 511v on the plates (too high) but when I switch it over to B+ it comes down to a more normal 440-ish plate voltage for these EL's. I'm using these cheap EL34's I grabbed off eBay (See that thread... here), and plugged four of them in to run these tests. Plate voltage across all four tubes is consistent, but the bias is across the range from 26-34-ish. So, I'm thinking if I can find two that have consistent plate voltage, and would bias at or near the higher end of 34-ish range with the current settings... I'm hoping there is enough room in the bias adjustment to get me to around 39-40ma. Am I correct in this line of thinking, or is there something that I'm missing that will cause a huge cloud of smoke and heartache for me?
 
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@War_in_D that 74mA at 425 is only about 31-32W idle plate dissipation which KT88s can handle easily and I'm assuming Scott knows that his power supply can handle that as well.

The idle current you want to target changes when the B+ voltage changes and the B+ voltage changes when the idle current changes. So you have do the whole measure/calculate/adjust routine and then measure and maybe do it all over again with the new numbers. You dial it in. Since EL34s are 25W tubes the equation is: 25W/plate voltage=max plate current. Then you multiply by .65 to get your target idle current (65% of max current AT THAT plate voltage). Then measure everything again and dial it in more. Once you are close, it only takes one or maybe two adjustments because the B+ won't really change for small idle current changes.

For EL34s/KT88s:
To measure bias voltage measure pin 5.
To measure plate voltage (should be very close to B+ voltage) measure pin 3.
To measure screen voltage measure pin 4.
You can find the pinout of your power tubes online and see what tube elements are connected to each pin.

I would only run those tubes at the low voltage setting. So if your plate voltage is 440vdc then: 25W/440v=0.057A or 57mA. 57*0.65=37mA. So you'd be targeting 37mA plate current at 440vdc plate voltage. The cathode current is the plate current plus the screen current which is around 5mA if your tubes are good and healthy. So you would target 42mA cathode current. I don't know what your bias meter reads, plate or cathode current.
 
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The Weber measures plate voltage, and then I was calculating based off the Weber bias calculator to come up with around 39ma at 440v at the plate. I think the Weber calculator figures at 70%, so that may be where the difference of the 2ma is at.
 
War_in_D said:
The Weber measures plate voltage, and then I was calculating based off the Weber bias calculator to come up with around 39ma at 440v at the plate. I think the Weber calculator figures at 70%, so that may be where the difference of the 2ma is at.
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It has to do more than that to measure current. To measure current the current either has to go thru the measuring device (which means both leads are at high voltage and ready to shock the fuck out of anything...or cause an arc-weldian spark that blows a fuse and possibly more...aka riskier OR it can measure that current across a known resistance because V=IR. I put 1 ohm resistors between the cathodes of my power tubes and their ground connection. So I just measure a tiny voltage, like say 42mV from the above 42mA example on pin 8 of my power tubes. That's also how all those 'external bias points' you see on some amps work. Safer and less risk to the amp.
 
SpiderWars said:
@War_in_D that 74mA at 425 is only about 31-32W idle plate dissipation which KT88s can handle easily and I'm assuming Scott knows that his power supply can handle that as well.
Click to expand...

Good to know, but as you had said before about having the B+ switch and the varying voltage... I think with all things being equal it would be better to just pick one way to run it, and then adjust everything to suit that selection.

SpiderWars said:
It has to do more than that to measure current. To measure current the current either has to go thru the measuring device (which means both leads are at high voltage and ready to shock the fuck out of anything...or cause an arc-weldian spark that blows a fuse and possibly more...aka riskier OR it can measure that current across a known resistance because V=IR. I put 1 ohm resistors between the cathodes of my power tubes and their ground connection. So I just measure a tiny voltage, like say 42mV from the above 42mA example on pin 8 of my power tubes. That's also how all those 'external bias points' you see on some amps work. Safer and less risk to the amp.
Click to expand...

Yeah, see.. now this kind of talk is getting way above my head. LOL However, I have heard of the 1 ohm resistor across the cathode to measure bias before. I've just never been one to try it, mainly because it would probably mean me having to install the resistor and I don't trust myself to do that. I'm not sure how the internals of the Weber are set up, but there is a switch that I flip that gives me plate voltage on one side and then bias on the other. Before I bought that, I had one of those cheap bias probes that you get from Eurotubes and just plugged it into a multimeter to check my bias. The problem with that was that I didn't have the actual plate voltage to calculate the dissapation so I would just have to figure it base on the assumed watts of the tube that I was using. I would love to know more about this stuff, but I just don't think my old brain is capable. LOL
 
Also, what is the voltage on that black wire? That's the supply voltage to the bias supply. Since the bias circuit is basically Marshall values, you should be able to estimate some resistor values for those switch resistors just by looking at some Marshall schematics. It looks like there is a bias winding for the bias supply so there is probably around 95vdc-100vdc there. If so, I think Marshall often used 27k there for EL34s but check that.

EDIT: Spoke too fast...I think that's a 47k resistor next to the bias pot and if so I think that's what Marshall used for 6550s which bias up more like KT88s.

And I just read something on the Weber Bias Rite that it does measure cathode current so figure about 5mA.
 
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Yeah, schematics might as well be Egyptian hyrogliphs to me. I have zero clue what I'm looking at, or what any of that stuff means. :unsure:

You have to remember that when it comes to all the EE/amp tech stuff beyond looking at a bias probe, I'm basically like a two year old stickin' a fork in a wall outlet. Nothing good is going to come of it. But given what I've seen with the plate voltage and bias readings I'm getting... At the end of the day, if I were to run the amp in B+ mode at around 440v and find a duet of EL34's that would bias up into the 37ma range, I should be good and theoretically it shouldn't fry anything right?
 
After it was all said and done.. stuck the KT88's back in, and biased it a cooler than it was. (49.5mv vs. 51mv)

I think the 88's give the Nitro enough of a tonal difference from the QR (HUGE bottom end), that it warrants just keeping it that way. Maybe enough of a difference, and close to Mesa territory that I could get rid of my Mesa. I'll have to play around with that some more though before making that call though.

@SpiderWars Appreciate all the help and insight! Thanks!
 
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