DIY 5F2A: Distortion, not loud. Where do I look?

heybluez74

TDPRI Member
Joined
Nov 29, 2022
Posts
20
Age
48
Location
Charlottesville, VA
I am working today so won't be able to get to this till later but I am planning on triple checking all my connections/wiring just to make sure I did not make a dumb mistake... more soon
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
9,019
Location
Canada
The transformer is rated for 600V @ 173mA, almost double the current usually used. Due to this there is less sag across the secondary winding causing the high voltage. Putting a 600R resistor in place of the 500R will cause less current but will also increase the voltage a little. So not the way to go. The heater voltage is 3.6 x 2 = 7.2V rather than the 6.3V. The winding is rated for 5A, you are drawing 1A.
 

heybluez74

TDPRI Member
Joined
Nov 29, 2022
Posts
20
Age
48
Location
Charlottesville, VA
The transformer is rated for 600V @ 173mA, almost double the current usually used. Due to this there is less sag across the secondary winding causing the high voltage. Putting a 600R resistor in place of the 500R will cause less current but will also increase the voltage a little. So not the way to go. The heater voltage is 3.6 x 2 = 7.2V rather than the 6.3V. The winding is rated for 5A, you are drawing 1A.
So best to get a proper PT?
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
9,019
Location
Canada
So best to get a proper PT?
Will you be using the current transformer for another amp? If not and you do not want to spend the money you can reduce the voltage by putting resistors in series with the heaters, zener diodes for the supply voltage.
 

heybluez74

TDPRI Member
Joined
Nov 29, 2022
Posts
20
Age
48
Location
Charlottesville, VA
I don't plan on using it for another amp (even though despite my current frustrations, building this amp is a lot of fun). I think I get the series of resistors layout but any tips/links re: using zener diodes for supply voltage?
 

heybluez74

TDPRI Member
Joined
Nov 29, 2022
Posts
20
Age
48
Location
Charlottesville, VA
The transformer is rated for 600V @ 173mA, almost double the current usually used. Due to this there is less sag across the secondary winding causing the high voltage. Putting a 600R resistor in place of the 500R will cause less current but will also increase the voltage a little. So not the way to go. The heater voltage is 3.6 x 2 = 7.2V rather than the 6.3V. The winding is rated for 5A, you are drawing 1A.
Curious -- when I look at the spec on the box of my Hammond 272FX it looks like it is rated for 600V 150m ADC, not 173mA. Where are you getting that from? (asking so I can learn)

Also, would I use the GRN/GRN-YEL to get 6.3V to the heater? Right now, using GRN-GRN.
 

dan40

Friend of Leo's
Joined
Aug 19, 2015
Posts
3,020
Location
Richmond Va
Curious -- when I look at the spec on the box of my Hammond 272FX it looks like it is rated for 600V 150m ADC, not 173mA. Where are you getting that from? (asking so I can learn)

When we look up the specs on the Hammond website, I get the same results that Printer is quoting. It's odd that your box says something different. It's possible that Hammond made a design change recently and your is an older model.

Also, would I use the GRN/GRN-YEL to get 6.3V to the heater? Right now, using GRN-GRN.

Your heater voltage is high because the filament winding has a very high current rating. It was intended to run a lot more tubes, or tubes with a higher current draw. Since you are only using 1 amp instead of the 5 amps that it's rated for, the voltage will tend to be higher than normal.

You should have both green wires running to the pilot light and tube sockets and the green/yellow centertap wire connected to ground. If the green/yellow is not connected to ground, this could also be causing your hum.
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
9,019
Location
Canada
Curious -- when I look at the spec on the box of my Hammond 272FX it looks like it is rated for 600V 150m ADC, not 173mA. Where are you getting that from? (asking so I can learn)

Also, would I use the GRN/GRN-YEL to get 6.3V to the heater? Right now, using GRN-GRN.

The windings each have their own resistance. As you draw current there is a voltage drop across the resistance (V = I x R). To deliver 6.3V at the spec'ed current of 5A the open circuit voltage has to be above 6.3V. Just for kicks let us say it is 7.5V with no load and 6.3V at 5A. At 2.5A it would roughly be 6.9V. Tubes usually can operate +/- 10% of 6.3V although they would rather be within 5%. As said above, the GY should be at ground potential, or at a higher dc voltage such as the capacitor across the 6V6 cathode resistor. This is called an elevated heater potential. Try grounding the wire. After you sort out the amp's operation and you still have some hum you can try elevating the wire.
 

peteb

Poster Extraordinaire
Joined
Apr 25, 2003
Posts
5,369
Location
Cascadia
As fender amps go, 300-0-300 is low. (tweedish?)

Putting a 600R resistor in place of the 500R will cause less current but will also increase the voltage a little.

the plate voltage is not too bad. The plate dissipation Is high and only too high for a tweed amp but not for a BF/SF champ which this amp is not.

he has

434 plate minus 24 cathode for 410 volts. High for tweed but not too high for a BF/SF champ.
 

peteb

Poster Extraordinaire
Joined
Apr 25, 2003
Posts
5,369
Location
Cascadia
your voltages, current and plate dissipation are all in range, although on the high side, especially for a tweed circuit.

being a little high will not cause

It 'works' but the output is distorted and weak.

unless the tube is no good.
 

heybluez74

TDPRI Member
Joined
Nov 29, 2022
Posts
20
Age
48
Location
Charlottesville, VA

Attachments

  • IMG_1509.jpeg
    IMG_1509.jpeg
    209 KB · Views: 14

heybluez74

TDPRI Member
Joined
Nov 29, 2022
Posts
20
Age
48
Location
Charlottesville, VA
your voltages, current and plate dissipation are all in range, although on the high side, especially for a tweed circuit.

being a little high will not cause



unless the tube is no good.
reversing the OT wires fixed the distortion and weakness. Now it is a hum/buzz.... also strange is that I can wave my hand over the preamp section (12AX7) and it will have an effect on the noise. Grounding issue?
 

heybluez74

TDPRI Member
Joined
Nov 29, 2022
Posts
20
Age
48
Location
Charlottesville, VA
Here is what it sounds like at this moment --

So def has power... just hum plus slight distortion... and what I would say slight chorus effect (even though I kind of dig it). Would you all still consider this a power supply issue?
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
9,019
Location
Canada
As fender amps go, 300-0-300 is low. (tweedish?)



the plate voltage is not too bad. The plate dissipation Is high and only too high for a tweed amp but not for a BF/SF champ which this amp is not.

he has

434 plate minus 24 cathode for 410 volts. High for tweed but not too high for a BF/SF champ.
With the tube dissipating 19W how do you suggest cooling it down?

v7MghKQ.png


A power supply of 410V is already high for SE, 434V is more so.
 

peteb

Poster Extraordinaire
Joined
Apr 25, 2003
Posts
5,369
Location
Cascadia
PT

RED — 688VAC
YELLOW - 5.65 VAC

thanks for clarifying.

688/2 = 344

344 * 1.414 = 486.4 VDC - 48.4 VDC = 438 VDC, the B+ voltage.

I calculate your 5Y3 drops 48 VDC. a little less than expected.
It might drop more voltage with a vintage 5Y3.

I agree with lower left coast.

I recently tested some 5Y3 tubes for voltage drop.

the chart says 60 volts. NOS tubes drop 80-90, old RCA tubes drop 110-120.

Hey Blues. The amp sounds good.
With the tube dissipating 19W how do you suggest cooling it down?

I would look for red plating in the dark and in the light.

if the red plating were minimal I would consider running the amp as is.

as lower left coast recommended, a rectifier tube change could help.

Putting a 600R resistor in place of the 500R will cause less current but will also increase the voltage a little.

this seems viable to me. The AA964 Princeton schematic shows 420 VDC on the plates. The OP has some wiggle room.

a combination of both would work really well.

lower the voltage with a different rectifier tube and then lower the current with a rebias.
 
Last edited:




Top