New 5F6A Build Issues

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Lummox
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New 5F6A Build Issues

Post by Lummox »

I just completed a 5F6A copy using Magnetic Components Iron, and a Mojotone chassis, and small parts kit. I got a bit carried away the other night, and kept soldering after I probably should have gone to bed. On the octal sockets, I hooked up the heater wires correctly, grounded pins 1 & 8 to the chassis, but from there, went backwards on the pinout. For example, I installed the screen resistor across pins 3 & 5 instead of 4 and 6, hooked the OT leads up to pin 6 instead of 3, and so on. Of course, when I powered it up, the screen resistors blew immediately, and I shut it down as quickly as I could.
I haven't had time until today to look at it, and I've moved everything to their correct pins, and double checked the rest of the amp, but before I proceed any further, I wanted to see if I've caused any permanent harm.
How should I proceed in checking it out from here ? Does anyone know of a good tutorial which might be of use ?
joeboo88
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Re: New 5F6A Build Issues

Post by joeboo88 »

Hi. If you look in the technical section, there is a new sticky. It is the first topic in the technical section.
It called RJ's build protocol, you are around step 8 give it a looksey.... good luck
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

I would start by powering up with no tubes installed and checking voltages on the power tube sockets, especially 3, 4, and 5. That should give you a pretty good indication of any collateral damage. Pin 3 should match the voltage at the rectifier's pin 8, and pin 4 should match the voltage at the standby switch. Also verify that you have sufficient negative voltage on pin 5 (-50V or so), and that the bias pot is working if you have installed one.
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

Thank you for your responses.

Without tubes, I get the following:

Pin 3 = -0.112 V DC
Pin 4 = -0.103 V DC
Pin 5 = - 57.14

Rectifier Pin 8 = -0.124

Stdby Sw. = -0.135 V DC
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

You'll need to have the rectifier tube installed to get reasonable pin 3 and 4 voltages. The negative voltage on pin 5 looks good. Do you have a way to adjust that or will you have to change a resistor?
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

I don't have a way to adjust it. I decided to build it stock, and then once I was happy with it, maybe add test points and a bias pot.

With a 5U4G rectifier tube, I get the following :

Pin 5 = -59.2

Pin 4 = 475 V DC

Pin 3 = 477 V DC

Rect. Pin 8 = 479 V DC

Stby = 480 V DC
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

Looks like everything is ok. You could add the power tubes now and see where they bias.
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

I'm getting -56.9 V DC on V1, and -57.04 on V2. Measuring pin 5.
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

So the power tubes are installed now? Can you measure current? You can use OT primary resistance or measure voltage at pin 1 if you have 1R resistors to ground.
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

Right. Which leads me to a question. I've seen versions of this build where pins 1 and 8 are grounded directly to chassis, versions where just pin 8 grounded to chassis, and versions grounding through a 1 ohm resistor. Along with this is usually the advice "don't do it any other way, my way is best".
So, what gives ?

Also, I'm going to be running a 4 ohm cabinet, using the 4 ohm tap, so instead of a 27k feedback resistor, I'll need either a 39k resistor, or tack on a 10k to the 27 k, correct ?
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Re: New 5F6A Build Issues

Post by pompeiisneaks »

you can still setup nfb on the 8 ohm tap, as it still sends signals there, you just wont get them to the main output via the 4 ohm ( or so I understand it, but I could be mistaken). As for the 1/8 pins and 1 ohm, the 6l6, 6v6, and others like it don't connect pin 1 so it doesn't matter, but the el34 and some others need it grounded, so it's always best to just connect the two together. AS for the 1 ohm, it just makes checking the tube's current easier. you need to use precision 1% 1 ohm resistors and then if you measure the mV at the top of the resistor, that can then easily be also treated as mA due to ohms law: I = V/R which is something divided by 1 or just itself. (of course in theory you should exactly measure the resistance, to be precise, but a .9 ohm resistor's value isn't going to make your estimate of plate current that far off. )

So basically, the 1 ohm isn't necessary, but makes biasing much more convenient.

~Phil
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

I installed 1 ohm, 3 watt wirewound resistors from 1 & 8 to chassis on both power tubes, and now I'm getting 14.1mV on V4, and 12.4 mV on V5.
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

Putting a 1R between pin 8 (cathode) and ground is a very convenient and safe way to get cathode current. External test jacks and an accessible bias trimmer are great if it's difficult to get inside your chassis. On a tweed Bassman, removing the upper back panel gives you full access so there isn't much need.

The pin 8-pin 1 connection depends on what tubes you are planning to run. 6L6 and 5881 have an internal cathode-suppressor grid connection so you don't need to tie pins 8 and 1. That connection is shown on the 5F6-A layout, but that was probably included as a ground for metal envelope 6L6's. Sometimes Fender used pin 1 as a tie point for a grid stopper since it is not connected. You can still add a grid stopper (5k1, say), just do it Marshall-style, soldering the resistor close to pin 5 with the lead wire soldered to the other end.

For the NFB, if you are using the 2-4-8 ohm secondary Mag Comp OT than you can connect to the 2-ohm tap through the standard 27k resistor and still use any of the three taps for the speaker out. Moving the NFB to the 4-ohm tap would require increasing the 27k to 38k to match FB signal; 39k is close enough.

What tubes are you running, and what plate voltage do you have now?
Last edited by martin manning on Sun Jul 23, 2017 12:44 am, edited 1 time in total.
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

The OT I'm using has taps for 4,8, and 16ohm, but will be used with a 4 ohm load.

The tubes are Sovtek 6L6WGC/5881, and I'm getting 443 V on the plates.
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

The Sovtek data sheet doesn't list Pa Max, but If I assume 23W (like a 5881), then I'd be looking for 60 to 70% of that, or 0.65*23W/443V = 34mA. You'll want to decrease the 56k in the bias supply.
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