Problem with new Twin build

[rhubarb9999]

Hey gang,

Well, I just fired up a Twin that I built from scratch using Tino Zottola's book.

The chassis and iron are recycled from an old Earth Sound Super Bass amp.

Because this amp was a 4 - 6L6GC, I figured that the PT and OT would suit a Twin build well.

So, I fire it up today with the tubes in and all my voltages are about 1/2 what they should be .. Pins 3,4,and 6 on the power tubes are all reading about 260v (the build guide says they should be 460v) Pin 1 on both preamp tubes is 156v .. guide says 260.

The Primaries out of the PT are 320v AC

Heaters are at 2.75v

I have a few theories.

1) When putting the diodes in series, the iron did get a little close. Is it possible to fry a diode from too much heat? Would this cause only one side of the PT to do it's thing .. cutting the voltage in 1/2?

2) If you look at the schematic below for the original amp, there is a blue wire that looks like it has the full 470v on it. I did not use that wire in the Twin ... should I have?

I currently have the R/Y and the Orange going to ground, both Red wires are going to the diodes and the greens are going to the heaters and pilot light.

I have not tried to run any audio though it yet, there is just a nasty hum when I power it up.

This is only my second build .. so I am new at this .. any help would be great!

Thanks

[Bob-I]

I don't know if you could've fried the diodes from heat, but it does sound like one side is not getting voltage.

I'd start by disconnecting everything from the diodes including the connection between the 2, then measure each one by itself. It could be a diode, a shorted cap, an incorrect balance resistor etc.

[Richie]

the blue wire in the schem you posted is the bias tap wire.. the 470 you see is probably the 470 ohm resistor value, not voltage.

Since your using the scavanged iron, i'd use the power supply hookups and info from the schematic. I don't know what info you have that you used from the other book.. or what twin this is. The later twins used a bridge recto which is a different setup than the one in the schematic you posted.

Whats the bias reading your getting?

[tele_player]

Since all the voltages are low, INCLUDING THE HEATERS (which don't involved the diodes), I'd first check the primary hookup.

[Phil_S]

Since all the voltages are low, INCLUDING THE HEATERS (which don't involved the diodes), I'd first check the primary hookup.

After this, I'd want to disconnect all the secondaries and see what it puts out in AC without a load. I'd check the bias tap and filaments, too.

Are you sure you grounded the CT and not the bias tap?

[rhubarb9999]

OK .. more info.

The voltage off the bias diode/resistor was -65v

First I found the 'Duh" moment .. I did not tie to first 2 filter caps together .. so only one side was getting juice ... duh.

OK .. fixed that. I turn the amp back on .. and before I could start to measure anything the power tubes were WAY to hot .. bright orange.

What are some common causes of this? I have attached the schematic that I am working from if that helps. This is supposed to be aAA270 chassis that has been blackfaced to AB763.

[rhubarb9999]

Another update :

I found a BIG typo in Tino's book. In the step by step instructions, he says to tie the 1500 ohm bias resistor between pins 5 and 8 .. instead of 5 and 1 .. then tie the bias voltage to pin 8!


So .. basically, I had no bias. That would cause the tubes to get a little warm

The schematic was correct (as was the AB763 layout) .. so it was easy enough to fix. Next time I will not just follow the directions blindly.

All my voltages are now spot on .. the heaters are a tad low at 2.85v but voltage across the tubes is around 470v .. bias is solid at -60v.

Next issue : I plugged in my guitar and heard some noise! But as soon as I hit the strings hard, I started to get this machine gun sound .. it also happens as I was probing the pre-amp tubes. This was when I first turned it on and the bias was around -70v. Once I backed it down to -60v the amp started wailing and howling. Should I try reversing the leads to the OT?

Done for tonight .. back at it in the AM

[paddy]

Hi there

Your heater voltage is way too low! It should be around 6.3 Volts AC.
Nothing will work correctly until you fix this problem.

Cheers
Paddy

[paulster]

Are you measuring your heater voltages between pins 4 (or 5) and 9 on the preamp tubes, or are you still referencing them to chassis ground?

If you've got the usual 2x100ohm virtual-centre tap going on or a real centre-tap in your filament winding then your heater voltages referenced to the chassis ground will be +/- 3.15Vac ideally.

It sounds like you're in the ballpark and might be measuring them wrong.

Paul

[rhubarb9999]

Correct, I was referencing the heater voltage to ground. Across the heaters I am getting 5.9v

So I reversed the OT leads and the Normal channel is now rockin' Clean, spanky, loud .. awesome stuff! The TMB and volume controls work .. as does the bright switch.

The vibrato channel .. not so much. there is sound, low, fuzzy. None of the controls work. I am going to go back and check all the wiring to the pots .. might be something wrong there. I put in a couple of switches to add some of the popular mods (switch out the vibrato and the tone stack). I think I will wire it back to stock and go from there.

But at least I know the power supply, power amp and one preamp are working .. it's a start

Wish me luck .. I'm going in!