5881 in a 5f2a

[jsnwhite619]

I did one a few years ago for someone, and I wasn't a fan of the bigger tube in there. But, I was trying for a Swiss Army knife that did everything ok. It was setup for 6v6 with the bigger tube option. If I remember correctly, I'm thinking that it ended up perfect for a 6v6, the OT impedance works for a bigger tube. But the bias was all for a 6v6.

Well, I feel like my 5f2a game is pretty dang stout these days, and I got the "more power" monkey on my back on a recent thread about a parallel SE build. So, I'm looking at a dedicated big bottle Tweed Princeton in a 5e3 sized cabinet. And I have a 5f2a chassis sitting here. And I have a 100mA power transformer that fits it. So, the cabinet is framed out, the board is punched, and I'm still waiting on a few more parts to float in.

I modified the board layout more toward what I like these days. Just a couple things that make more sense to me. One is moving the bias resistor to the end of the board like a 5e3. I am making one big change though. I used the Princeton Reverb style NFB in my College Dropout I built a few months ago and loved it, and I noticed that the Blackface Champ used the same - 2700R with a 47R tail and a second gain stage. So, I'm doing the same, except just using the 5f2a tone knob instead of the B/T tone stack. Also will be putting a 2 option switch on the NFB. I have 5W dropping resistors on the way and 10W bias resistors just because that seemed an area of concern in most of the threads I researched. Speaking of bias, I will have a selection of 250R, 270R, 300R, 360R, 400R, 470R, and 500R resistors to choose from. I've seen everything from the lowest to highest listed and recommended online, so, since the resistors cost less then shipping, I went ahead and got a bundle to work with.

Transformers will be Hammond 290AX 100mA 275v/325v PT, and the ClassicTone 15W OT with 5k/8k options. My plan is to bias it for max with a GZ34, then let the rectifier options (3A PT) be available to change the tone & feel instead of changing the power tube to 6v6.

Any thoughts or input is welcomed, especially if it's from experience and will save me trouble down the road.

 

[Lowerleftcoast]

The 275v tap with a GZ34 will probably get to ~360 B+
Single ended amps are a little noisy , so maybe a choke to handle at least 100mA. Similar to 5E1 schematic.
The 5881 will like a 2.5k OT primary, so maybe run the 5k option with 4ohm load on the 8ohm tap. (2x10?)

 

[jsnwhite619]

The 275v tap with a GZ34 will probably get to ~360 B+
Single ended amps are a little noisy , so maybe a choke to handle at least 100mA. Similar to 5E1 schematic.
The 5881 will like a 2.5k OT primary, so maybe run the 5k option with 4ohm load on the 8ohm tap. (2x10?)

I've gotten my 6v6 versions pretty silent up to 10 on the dial, so I'm not worried about noise. I only use JJ's, so I was thinking bumping it up to 400v+ for the hell of it. If the 6v6 is good to 500v, I'm not going to baby it.

 

[James Knox]

I've gotten my 6v6 versions pretty silent up to 10 on the dial, so I'm not worried about noise. I only use JJ's, so I was thinking bumping it up to 400v+ for the hell of it. If the 6v6 is good to 500v, I'm not going to baby it.

“...make it take it...” muchxs

 

[Kevin Wolfe]

Following this one @jsnwhite619. I’ve been jonesing to do a fatter SE next. Torn between beefing up a 5f2a and/or a 6L6 Vibro Champ. Been listening to clips of the Allen Chihuahua. That little booger sounds great to have a SS rectifier.

 

[jsnwhite619]

It's turning into a long wait for parts to drift in for this one. One question - what would be advised wattage rating for the dropping resistors? I use 2W metal oxide as a general rule for everything. I ordered 5W but they are somewhere between here and the internet still. What's the vote here? Would 2W be sufficient, or wait it out for the 5W? I plan on running things as high as I can get away with.

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[Nickfl]

It's turning into a long wait for parts to drift in for this one. One question - what would be advised wattage rating for the dropping resistors? I use 2W metal oxide as a general rule for everything. I ordered 5W but they are somewhere between here and the internet still. What's the vote here? Would 2W be sufficient, or wait it out for the 5W? I plan on running things as high as I can get away with.

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2 watt should be find. That first dropping resistor is just getting the current from the power tube screen and the preamp tube, so we're probably talking about half a watt, maybe 1 watt under max signal. The second dropping resistor is just getting the 12ax7 plates, so less than 1/10th of a watt at idle, even a half watt resistor would be fine there.

 

[jsnwhite619]

I figured it would be, but there turns out to be a lot more speculation than fact online when it comes to souped-up Champ-type builds. This was one of those areas where I've seen the suggestions range from "I left them stock" to "should probably increase to 5 watts".

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[jsnwhite619]

Well, for future reference, if you click to expands Mouser's shipping options and see the cheapest economy option down toward the bottom, they will teach you a lesson for picking it! Ordered 15 resistors on 7/8...THEIR local UPS in TX finally received it yesterday with est. delivery for 7/20. $3.50 saved on shipping, 12 days to get the order. I'm going to call this one a draw!

 

[sds1]

Best way to shop at Mouser is to just load up your cart until their $8 2-day shipping feels like a solid deal.

For me that's around the $75 mark.

 

[jsnwhite619]

My OT is arriving today, so getting closer. I'd like some feedback on what I'm thinking for the OT setup.

If I use the 325v PT tap & 5u4, I'm estimating approx. 415v-420v B+. Using Zout = Va/(Pa/Va), with the 5881, I'm getting in the neighborhood of a 7.5k primary. The OT has 5k and 8k options, I had always assumed to just use the 5k for 5881, but it looks like that would only be true if using a much lower voltage in the 350v range. Am I looking at all this correctly? I'm also estimating 380v for a 5y3, and that works out (I think) to 6.2k, and a GZ34 with 440v and 8.4K.

 

[jsnwhite619]

I forgot how big these OT's are! It's on the left, a Tweed Deluxe OT on the right.

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[elpico]

My OT is arriving today, so getting closer. I'd like some feedback on what I'm thinking for the OT setup.

If I use the 325v PT tap & 5u4, I'm estimating approx. 415v-420v B+. Using Zout = Va/(Pa/Va), with the 5881, I'm getting in the neighborhood of a 7.5k primary. The OT has 5k and 8k options, I had always assumed to just use the 5k for 5881, but it looks like that would only be true if using a much lower voltage in the 350v range. Am I looking at all this correctly? I'm also estimating 380v for a 5y3, and that works out (I think) to 6.2k, and a GZ34 with 440v and 8.4K.

5k should make a little more power than 8k, but I'd think the one that sounds better is more important. Probably only a watt or two difference between the two loads.

 

[jsnwhite619]

Long week and weekend, but I finally got parts in and got a little bit of work done this afternoon & tonight. As you can see with my resistor lineup, I don't plan on "good enough" when this thing fires up. That's pretty much every 10W resistor I could get between 250R and 500R, so I ought to be able to get things dialed in right for the 5881.

 

[jsnwhite619]

Well, I don't think I was fully realizing what a single 5881 is going to do here. Already after midnight, so other than turning it up enough to check that it works, I can't do a whole lot tonight. The NFB switch should prove interesting when I get a chance to crank it up tomorrow. Even at low volume it seems to have a very noticeable effect on the tone, and I'm nowhere near breakup levels, not by a long shot. But, I have the feeling it's going to be pretty dang loud.

I went middle of the road with the bias resistor and started with 400R with a 50uf/50v bypass capacitor; I am using the 8k OT primary right now. With a 5u4 and 5881 (both JJ) measuring and using Rob's calculator I get

• Approx 390v B+ (can't remember specific number)
• 349v Plate/Cathode
• 61mA Cathode current (per the calculator)
• Plate dissipation 20.1 watts
• Plate dissipation 77%

Obviously, it can go hotter, but I'll give it some playing time first for a comparison later. If anyone sees something that jumps out at them let me know. This is the first 5881 I've ever even bought before, so all new to me.

Before I quit for the night, I just went back out and tried a JJ 5y3 & 6v6, just to measure. 113% plate dissipation, and funny enough, the exact same 349v plate/cathode measurement. So, not out of the ballpark to have an all-in-one as it stands.

 

[jsnwhite619]

 

[Kevin Wolfe]

Oh, it’s gonna be loud. Should be punishingly loud by the time it breaks up. But for SE operation you probably need that plate diss. to be up around, or even above 100%.

 

[jsnwhite619]

Well, I changed the resistor from 400R to 360R, and the voltage changed lower, but in the end the bias dissipation wattage/% levels came out to exactly the same point @ 77% and 20 watts - ?? I can't remember where the GZ34 measured out to - maybe around 90% and 415v B+ - and really cleaned things up. 5y3 sounds pretty freakin' sweet too if you don't want any real clean headroom.

I messed around a bit this afternoon and got enough recorded to pick through for a quick demo clip. I've lowered the second gain stage bypass from 25uf to 2uf since this clip and cut some of the mud on the chords and lower stings. This thing can rock out though! With my Nashville Tele through the Legend 1058.

 

[James Knox]

Well, I changed the resistor from 400R to 360R, and the voltage changed lower, but in the end the bias dissipation wattage/% levels came out to exactly the same point @ 77% and 20 watts - ?? I can't remember where the GZ34 measured out to - maybe around 90% and 415v B+ - and really cleaned things up. 5y3 sounds pretty freakin' sweet too if you don't want any real clean headroom.

I messed around a bit this afternoon and got enough recorded to pick through for a quick demo clip. I've lowered the second gain stage bypass from 25uf to 2uf since this clip and cut some of the mud on the chords and lower stings. This thing can rock out though! With my Nashville Tele through the Legend 1058.

Sounds treat J!

How did you Record it?

 

[jsnwhite619]

SM57 into a desktop digital interface. Audacity is a great free download to use. I actually recorded in Audacity, then used Pro Tools to piece it together and boost the level a bit.