New 5E7 Bandmaster build

[powdog562]

Hi all. Just finished a 5E7 Bandmaster that sounds great until I turn up the volume and hit a hard chord, then a "chirpy" garbled sound that diminishes as the chord does. Sounds like a cold solder joint, but can't replicate it by banging on the amp. Swapped out tubes with no change. 345 VDC on the plates, biased at 40 mA. Ideas? Thanks.

 

[Blue Strat]

Schematic of what you built? Photos?

345V on the plates obviously isn't right. Disconnect everything down stream from the power tubes from the power supply. If the plate voltage increases to where it should be, you know what to look at.

 

[mark norwine]

well...maybe.

the 5E7 is a somewhat unique amp in that the plates are post-choke. If an incorrect choke was specified, there's no telling what sort of Vdrop is occurring.

For that matter, we don't know anything about the PT that was used. Low Vac sec to begin with? Low VA which is sloping the VAC down when current is drawn?

There's simply not enough info here to make even a reasonable guess as to the issues(s)...

 

[Blue Strat]

well...maybe.

the 5E7 is a somewhat unique amp in that the plates are post-choke. If an incorrect choke was specified, there's no telling what sort of Vdrop is occurring.

For that matter, we don't know anything about the PT that was used. Low Vac sec to begin with? Low VA which is sloping the VAC down when current is drawn?

There's simply not enough info here to make even a reasonable guess as to the issues(s)...

Yes, I agree about the PT. If disconnecting the rest of the amp from the power supply doesn't bring the voltage up, next stop is the PT and/or choke. What actual components are being used for these?

Bottom line: It's hard enough to troubleshoot amps over the web with complete information. I have a feeling we're missing LOTS of information on this one.

 

[mark norwine]

I have a feeling we're missing LOTS of information on this one.

This one?

I get that feeling on most threads!

 

[Blue Strat]

This one?

I get that feeling on most threads!

ROFL! Well, you've "only" got proof on all the other ones. On this one we only have speculation...so far.

With the appearance of the consensus that amps can be fixed over the internet with 2% of the information needed, it's no wonder that so many people believe that techs are overpaid.

 

[powdog562]

Good morning gentleman. Sorry for the obvious typo...I hastily posted this at the end of a brutal graveyard shift. I was pretty tired. I apologize.

The plate voltage is 435 VDC, not 335. I'm using a Heyboer P.T. 370-0-370, a Weber choke rated at 5-8 H @ 200 mA, and a Heyboer TO40 O.T. from Dave Allen. I really haven't had time to go thru the amp carefully yet. I had just finished bringing the amp up on a Variac when I noticed the problem and then left for work. The B+ is 447 before the choke and 437 after. I'll pour through the amp more carefully this afternoon.

 

[Blue Strat]

Clean schematic, highlighter, amp components and connections, DMM set to resistance...

That's how it's done, for starters.

 

[powdog562]

Good morning gents. Fixed the bug, but I'm a little worried about the excessive B+. Using this Heyboer HY022798 PT, I'm getting 470vdc at the first filter cap, 375 to the preamp plate resistors and 455vdc at the 6L6 plates.

V1. Pin 1 201vdc
Pin 6 202vdc
V2. Pin 1 205vdc
Pin 6. 320vdc
V3. Pin 1. 250vdc
Pin 6 308vdc

Do these seem scary high? I could use a Zener on the B+ center tap or just swap out a Hammond P-T290DX for the Heyboer. Thoughts? Thanks!

 

[Blue Strat]

Well, those voltages are definitely not correct for a 5E7. What rectifier tube are you using? If it's anything other than a 5U4, use the correct rectifier (a 5U4). If it's a solid state or 5AR4 that's why the voltage is too high.

The power tubes will survive at 455V but I don't recall ever seeing 375 on a preamp.

I'll most likely work but sound more like a Super Reverb than a 5E7.

Still getting 40mA idle current on the power tubes?

 

[powdog562]

I still have 40mA on the power tubes and I am using a 5U4. Tried a few different ones with the same result.

 

[Clutch21286]

Check the ground on the speaker out.

 

[Blue Strat]

I still have 40mA on the power tubes and I am using a 5U4. Tried a few different ones with the same result.

Sounds like the wrong power transformer for the job.

How does it sound?

 

[Trout]

I used the Hammond 290DX in the last 4 bandmaster I played with, 5U4 rectifier, they always come within 5% of vintage spec.
http://www.hammondmfg.com/pdf/EDB290DX.pdf
325-0-325 / 230ma

The parent company of Classictone also sells a similar unit, CE and AES sell them
P-TF22798
325-0-325 / 200ma

I have never experienced much difference as far as the small ma difference between the 2.
That Heyboer unit,,, was it for a Bandmaster Reverb maybe?
That calls for a 375-0-375

 

[Blue Strat]

I couldn't find an approved spec on the HY022798 but someone on another forum says it's a 370-0-370 secondary.

370 X 1.414 = 523 Subtract about 35 volts for a 5U4 voltage drop, and you're still close to 500V. Wrong transformer for a 5E7, though it will work and give massive headroom.

You'd want a 325-0-325 to get close to what a 5E7 should be.



325 X 1.414 = 460 Subtract about 35 and get about 425. Right on target.

 

[Trout]

I couldn't find an approved spec on the HY022798 but someone on another forum says it's a 370-0-370 secondary.

370 X 1.414 = 523 Subtract about 35 volts for a 5U4 voltage drop, and you're still close to 500V. Wrong transformer for a 5E7, though it will work and give massive headroom.

You'd want a 325-0-325 to get close to what a 5E7 should be.



325 X 1.414 = 460 Subtract about 35 and get about 425. Right on target.

That is really a sketchy setup as it sits. Assuming he used the 16uf/450V caps there is no voltage margin on the electrolytic caps at all. Yeah, you can get by but if the tolerance on the cap is off much I would expect issues.

 

[Blue Strat]

^^^ Ouch! True. Has he mentioned the voltage ratings? For Atoms, and I think F&Ts, the rating would be 475V, but that's still too close for comfort.

 

[Prairie Dawg]

I would probably put a zener diode in there and drop that voltage down to a more tolerable level.

 

[powdog562]

I use a Variac to form the electrolytics on new builds, run it up to 120 v. All of my measurements were unfortunately made with the amp still plugged into the Variac set at 120 v. When I finally plugged the amp directly into the wall outlet, the voltages all fell about 30 volts across the amp, bringing it to within 10 volts at all points. Live and learn.

 

[zenas]

Like an idiot when I built my 5E7 I went to the Classic Tone site and ordered the PT they reccomend for the tweed Bandmaster. http://www.classictone.net/40-18094.pdf
It's 355V-355V or 305V - 305V. In other words it's too dammed high one way and too dammed low the other.
Ended up going with the low voltage taps and a GZ34 to get close to Fender voltage but it' still a bit low and I sure didn't want a GZ34. Not that I don't like GZ34s just that 5u4s are cheap NOS.

Had I done my own research I would'a got this one. http://www.classictone.net/40-18073.pdf

Amp works fine but just ticks me off.