6G2 build: PT and OT suitability (harvested from an EL84 amp)

[mattosul]

Hi there (long time fan of the page and the good folk who give out charitable advice to idiots like me!),

was wondering whether I could use a power transformer with the specs listed below on a 6G2 Princeton build (6V6)?

The below PT, and the OT, are from a Harley benton GA15 tube amp (15watt, dual EL84)

12V-0-12V
6.3V-0
270V-0

see attached build layout I am going to use - 5v demanded by 5Y3 rectifier? can I use a voltage regulator to step down the 12v secondary to 5v using an LM7805?

should the OT be ok to use with 6V6's?

Is 270V as a high voltage AC current ok for a 6G2? The classictone PT in the layout says '275v-0-275v' so I thought close enough?

Kind regards,

Matt (Dublin, Ire)

[DeepBias]

Hi Matt,

You cannot directly use a 5Y3GT with a non-center-tapped transformer because it's a full-wave rectifier. You could replace the 5Y3 with full-wave diodes bridge rectifier but this will yield an higher B+ you will have to deal with.

5Y3GT heater is directly heated cathode, requiring 5V AC supply rated at about 3A for proper heating and rectifier operation.

The GA15 OT will be good to use with 6V6GT.

[dogburn]

I built a 6g2 using an old PT from a Voice of Music record player, which was 274-0-274, and it turned out great. Amazingly, the B+ voltages were really close to Fender specs. Your PT should work voltage-wise. Like DeepBias noted, without a center tap for the high voltage secondary, you can't use a 5Y3 with the standard circuit. If you go with the full-wave bridge rectifier, then the higher voltages shouldn't really be a problem.

The OT should work but looks like it would be something like 3.2k/8 Ohms, whereas 6g2 has ca. 8k/8ohms - so you could hook up a 4 Ohm speaker to the 8 Ohm tap and be close to the ratio you need (unless I got my math backwards).

[DeepBias]

The OT should work but looks like it would be something like 3.2k/8 Ohms, whereas 6g2 has ca. 8k/8ohms - so you could hook up a 4 Ohm speaker to the 8 Ohm tap and be close to the ratio you need (unless I got my math backwards).

Correct, lowering speaker impedance at secondary increases the primary impedance that the output tubes "see". You can also inject a small AC voltage (ie 12V AC) at the primary OT and then mesure the voltage at secondary to get the turn ratio for each tap, then you will know which tap / speaker impedance to use to be as closest as possible to the ideal 8K primary.

[mattosul]

Hi Matt,

You cannot directly use a 5Y3GT with a non-center-tapped transformer because it's a full-wave rectifier. You could replace the 5Y3 with full-wave diodes bridge rectifier but this will yield an higher B+ you will have to deal with.

5Y3GT heater is directly heated cathode, requiring 5V AC supply rated at about 3A for proper heating and rectifier operation.

The GA15 OT will be good to use with 6V6GT.

Thanks for your help. I will build a 4x diode (1N4007) FW diode bridge rectifier then!

[mattosul]

I built a 6g2 using an old PT from a Voice of Music record player, which was 274-0-274, and it turned out great. Amazingly, the B+ voltages were really close to Fender specs. Your PT should work voltage-wise. Like DeepBias noted, without a center tap for the high voltage secondary, you can't use a 5Y3 with the standard circuit. If you go with the full-wave bridge rectifier, then the higher voltages shouldn't really be a problem.

The OT should work but looks like it would be something like 3.2k/8 Ohms, whereas 6g2 has ca. 8k/8ohms - so you could hook up a 4 Ohm speaker to the 8 Ohm tap and be close to the ratio you need (unless I got my math backwards).

I will have to buy a 4ohm for it...if the signal ever reaches a speaker ;)

[mattosul]

The OT should work but looks like it would be something like 3.2k/8 Ohms, whereas 6g2 has ca. 8k/8ohms - so you could hook up a 4 Ohm speaker to the 8 Ohm tap and be close to the ratio you need (unless I got my math backwards).

Correct, lowering speaker impedance at secondary increases the primary impedance that the output tubes "see". You can also inject a small AC voltage (ie 12V AC) at the primary OT and then mesure the voltage at secondary to get the turn ratio for each tap, then you will know which tap / speaker impedance to use to be as closest as possible to the ideal 8K primary.

you think a FW bridge rectifier needs a capacitor and a resistor on it as well? Or is it just 4xdiodes? Sorry, online resources are all different? Thanks in advance

[SEL49]

Correct, lowering speaker impedance at secondary increases the primary impedance that the output tubes "see".

That's backwards. Lowering speaker impedance at secondary decreases the primary impedance that the output tubes "see".

[mountainhick]

Correct, lowering speaker impedance at secondary increases the primary impedance that the output tubes "see".

Nope

Lowering speaker impedance at secondary decreases the primary impedance that the output tubes "see".

Yep, and the ratio is the same, i.e. if you switch to half the speaker impedance, there is half the relative impedance at the OT primary, and vice versa... if you double the speaker impedance, it doubles the impedance at the OT primary

  Edit: Old brain brain fart. Corrected the above, meant to say the primary impedance of the OT changes by the same ratio same direction i.e increase or decrease.

[DeepBias]

Correct, lowering speaker impedance at secondary increases the primary impedance that the output tubes "see".

That's backwards. Lowering speaker impedance at secondary decreases the primary impedance that the output tubes "see".

Thank you for pointing out the mistake.    I’ll make sure to remember that.

Yep, and the ratio is the same, i.e. if you switch to half the speaker impedance, there is half the relative impedance on the OT secondary, and vice versa... if you double the speaker impedance, it doubles the impedance on the OT secondary

That makes total sense!

@Matt : Just replace the 5Y3GT with a full wave bridge rectifier, the rest on the circuit doesn't change. But you may have to add a resistor to reduce B+ voltage.

[kagliostro]

Give a look to the EZ81 tube spec, I think you have the solution

....... EDIT: May be or may be not, the ga15 has 2.1A @ 6.3v and the 6g2 require 1.5A but EZ81 consumption is 1A

So your tube is an EZ80

Franco

[HotBluePlates]

Give a look to the EZ81 tube spec, I think you have the solution

....... EDIT: May be or may be not ... So your tube is an EZ80 ...

There's no center-tap on the high-voltage winding, according to the Harley Benton GA15 schematic.  So using a tube rectifier would require at least 3 separate tubes, due to the bridge rectifier arrangement.

[kagliostro]

Why don't use an Hybrid Rectifier?

Franco

[DeepBias]

So your tube is an EZ80

Franco

Why don't use an Hybrid Rectifier?

Franco

EZ80 is rated at 80mA, would it be limit for 6V6GT PP, and the sum of 6g2 tubes heater + the rectifier would be around 2.1A, the GA15 heater tap is rated at 2.1A, but maybe he might want to see what guts the transformer has, who knows what people can do for that spongier compressed sag feel of a rectifier tube 
 

[kagliostro]

EZ80 spec Say

Ratings
6.3v @ 0.6A
PIV=980V
VhkMax 500V
IkMax 270 mA

Application Data
RMSin 350-0-350 V
Irect 90 mA
Cout 50 uF

Actually an EZ81 will be better but It requires 1 A @ 6.3 and I don't know if the PT Is a bit over rated respect the use on the GA15


May be you can use It or not, give a try and seenif the PT heat rises too much

Franco