First post - TubeDepot 5E3+ Tweed build and stuff

[Mike6158]

I've been doing a little reading here and I've decided to ask a few questions. A few years ago I bought a 5E3+ kit from TubeDepot. I've since built one of their Marshall kits (head only). Last winter I finally got around to building the 5E3+ (the plus is their designation).

I've been working on tube gear for a while, 10 years or so I'd guess, so still a newb. Prior to contracting Ilikebuildingguitaramps disease I worked on Ham radio, AM radio, FM radio, and lots of little circuit board projects (I have a board mill so prototyping can be quick, or incendiary), still do.

I restored a '66 Blackface Fender Bandmaster before building the Tweed, and before the Bandmaster I restored my favorite little amp, a Gretsch G6151T. . I moved away from buying kits after the Marshall kit. It sounds good but I don't have a cab for it and the test cab was a Silvertone XL??? cab that I plan to use with the 1484 that I am almost finished repairing. I have the parts for 4 more Fender style amps (Hoffman eyelet boards) to build. I like tubes, they teach me things. I'd also add that Televisions used to smell nice when they were on and were much more interesting than today.

Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar. The amp actually sounds very good (to my less than sensitive ears).

Let's get to the point, I think the amp is biased hot, like very hot. If it's not I'd appreciate an explanation for why not.

I built it cathode biased. I can mod it for fixed bias. I can also switch a jumper for negative feedback. Right now the jumper is "out". I'm using a EuroTubes bias checker (because I'm lazy and having my hand in my pocket is distracting).

48.4mA
388VDC

No matter how many times I put the numbers into the calculator I get 18.78W

Bias resistors are 220kΩ and verified
Speaker (not that it matters unless it's not plugged in): Jensen MOD 12/50 8Ω

The two JJ 6V6's current draw isn't far apart, about 0.1mA. It's been like this since I built it. I didn't notice it at first because I looked at 6L6 data when I was checking it (derp). The photo below is from the initial power up and nothing has changed. The test leads went to a Fluke meter to check voltage against the EuroTubes widget. It matched:



It has a circuit board (not a fan) and I've checked every component numerous times. The components all match the schematic and layout . And the amp sounds good. Loud... I can't crank it much. Even at 2 it takes very little guitar volume to make it neighbor unfriendly with just a little twist on the guitar volume knob. Like I said, I think it's biased hot but it matches the schematic. I guess I should include that...

At the moment my photo hosting site isn't working (again) so I can't upload a finished shot of the board. If that's necessary I'll keep trying.

Thoughts?

PS- Here's the before of the tops side of the Silvertone 1484 chassis. This is the nice part )



Front



After a little cleaning

[The Dude]

Welcome to the place!

Since 2 tubes share a common cathode resistor, you need to divide your current or voltage (depending on method) reading by 2. Also, don't forget to subtract your cathode voltage from the plate voltage to get the actual voltage across the tube.

[g1]

Since 2 tubes share a common cathode resistor, you need to divide your current or voltage (depending on method) reading by 2. Also, don't forget to subtract your cathode voltage from the plate voltage to get the actual voltage across the tube.

I think his bias probe goes in-line with one tube, so the current should be for just one tube? Not sure about this particular bias checker though.

Attached is their schematic. I think it probably just runs hot, and could be cooled by raising the value of the cathode resistor.

[The Dude]

Well, if that's the case, ignore what I said and I agree with g1 about raising the value of the cathode resistor. I thought we might be just measuring the voltage drop across the cathode resistor and calculating current from there.

Edit: I see the bias checkers now. When I first replied, the pics weren't there for some reason?

[Justin Thomas]

Loking at that schematic, I'd expect ~475V B+ with just the rectifier installed... I wouldn't expect 2x6V6 to drop nearly 100V from current draw without being biased crazy-hot. I'd raise the cathode resistor AND use a 5Y3 or 5R4 or something with a bigger voltage drop to lower that raw B+. But that's just me.

Justin

[Mike6158]

Testing... None of my replies have made it. Will this one? Well f

The rectifier is a GZ34

Correct, the EuroTubes bias checker measures actual DC voltage (pin 8 is ground) and only for the tube that's plugged in.

I was looking at the grid leak resistors when I wrote 220kΩ, it's actually a 250Ω 10W resistor.

I typed a really long reply and lost it. I won't quote the EuroTubes instructions again. Basically they say that for cathode biased amps the EuroTube widget measures actual DC voltage and subtraction isn't necessary.


Hopefully this reply will actually post. This is try number 6 or 7

[Justin Thomas]

I think what may be going on is the "new member screening process." I believe that until you are established there's a time wait on pics & replies. Be patient, it'll all come through.

Dude, I've had that picture-loading issue several times on my phone; even on guys like Juan, so I think that may be a separate issue. Usually going Back & then re-entering the thread fixes it.

Justin

[Justin Thomas]

Also: looking at your bias probe & meter, I see no way that the instrument can take cathode voltages into account. Yes, it measures plate current directly, and plate voltage directly, but we have no idea what your cathode voltage is, which is necessary when calculating the plate dissipation. Because the voltage used in calculating power is (plate minus cathode).

Unless it's built in to measure between the plate and cathode from the get-go. But in reading the instructions I didn't see that mentioned anywhere. Nearly everything in their literature seemed to "assume" a fixed- or grid-biased amp.

That's just how I read their instructions, anyway...

Justin

[The Dude]

Well if the probe itself measures plate voltage, it would almost have to reference to cathode and not ground. If that's the case, you wouldn't need to know cathode voltage or even cathode resistor value for calculations. We're measuring current directly.

[Justin Thomas]

Point taken.

I guess I'm never satisfied until I've ripped something apart to see for myself...

Justin

[alexradium]

Yes,do not use gz34,use a 5y3 but not a Sovtek.
If the current is still high,try a 330 or 390 ohm.

[Dave H]

I'm using a EuroTubes bias checker (because I'm lazy and having my hand in my pocket is distracting).

48.4mA
388VDC

No matter how many times I put the numbers into the calculator I get 18.78W

Does the bias checker really measure plate current or is it measuring cathode current?
If it's cathode current you should knock off about 10% in the calculation for screen current which would bring it down to 'only' 16.9W.

[Mike6158]

I have voltages and other info but this site keeps resetting and showing the message is too short. If I can't post there's not much point in continuing here...

and of course this one went thru

All voltages measured with a DVM not a VTVM. I have a perfectly good VTVM sitting on a shelf in the wrong state.

HV - nothing but rectifier tube: 428VDC.

Voltages measured to ground with all tubes installed

HV=428V---R21---389V---R22---299V (the 6V6's aren't dropping 100V, R21 is dropping about 40V, the output transformer is dropping the rest)

[The Dude]

The output transformer will drop very little DC voltage. To DC, it's just a piece of wire. If B+ is 428V (referenced to ground) and your bias probe tells you plate to cathode voltage is 388, that would mean your cathode voltage is somewhere around 40V, which seems way high. Can you measure cathode voltage and tell us what it is? Also, what is the value of your cathode resistor? Is it 250 ohms like the schematic shows? There are some things not jiving here.

[Mike6158]

Center tap to plate on the output transformer is 215Ω and 216Ω respectively. The output transformer is dropping about 11V per tube. Both Cathodes (to ground) are 27.3V. All measurements made with a DVM to ground. Pin 3: 413V, Pin 4: 384V, Pin 5: 0.16V; Pin 8: 27.3V