Heater voltage too high

[chunkitup]

I just built an amp with a Hammond PT and I am getting 7.1Vac for my heaters. I used 2 big diodes to drop voltage but my question is this. Besides shortening the life of the tube what would the sound difference (of the amp as a whole) be running the voltage that high?

[pdf64]

Is that voltage measured with the amp ready to operate?

[bob p]

Is that a hammond that has the 117VAC primary? Those things pretty much demand a bucking transformer in front of them.

even with a 117VAC primary spec, your voltages seem really high. is the PT loaded or not?

[Justin Thomas]

What Bob said. Tubes in or out? All my Fenders put out 7V with no tubes in, except for the Super Twin...

Justin

[chunkitup]

Tubes in and the amp fully turned on. The tranny is 270fx and I am using the wht-grey PRI taps which state 115v. My line voltage is consistently 120v. I just wondered how the amp would sound if I didn't drop some of the heater voltage. Come to think of it I bet my PS for the relay is also affected.

[bob p]

There could be several things going on here. You could have too much current rating in your transformer for the amp that you've built. Insufficient load could result in high voltages. It would help to know about the circuit. Without knowing the load we're taking shots in the dark.

That said, I'd hook up the white/black leads and see if the heater comes down to about 6.5V.

[Tom Phillips]

... I just wondered how the amp would sound if I didn't drop some of the heater voltage...

Based on the classic literature you may hear a little more hum with the higher heater voltage. I'd guess that there would be minimal impact with heater voltage up to 10% high (6.93V). Higher than that and tube life would likely be shortened. How much is hard to say since we don't get actual specs on the tubes we buy now.

I thought Hammond had started supplying transformers with 120V primaries. No?

[bob p]

AFAIK Hammond has offered 120V taps in the 300 series for a long long time. But the price premium for those is pretty high. They recently started replacing the 117 VAC primary on the 270 series with 115 and 125 taps. Those are quite a bit cheaper than the 300 series.

http://www.hammondmfg.com/pdf/EDB270FX.pdf

[chunkitup]

The amp is a 20w PP 6v6 power amp with a standard marshall preamp, 3 gain stages + CF. Change to the 125v pri tap is a option I havent thought about.

[bob p]

I'm not sure exactly what you mean by "standard marshall preamp." Some marshalls have lots of tubes in them. Do you mean 3 small bottles, like a Tweed Bassman / JTM45?

Unfortunately, there are still a lot of missing pieces to the puzzle.

Here's what we know:

* heater voltage

Here's what we don't know:

* tube compliment / load
* desired B+
* actual B+
* type of rectifier
* voltage on 5V tap

With the data we have, there's no way to tell if the voltages are too high across the board, or if they're just too high on the heaters. There are lots of ways to tackle the voltage problem, but without knowing more about the amp we're just shooting in the dark. It would really help to know if the voltages are high across the board, or if they're just high on the 6.3volt tap and they're OK everywhere else.

[J M Fahey]

As a side note, I absolutely *hate* diodes for dropping AC voltages, much more in filaments.
We are messing a sinewave, adding buzz where there was none, etc.
Sort of sleeping with the enemy.
Calculate and use the small series resistor you'll need for that slight voltage drop.
That, if you don't solve it the proper way, using a 120V primary.

[chunkitup]

Here you go bob.
2- TungSol 6v6gt PP power amp
3- TungSol 12ax7 (JCM800 style-3 gain stages and a CF into TS to MV then off to the PI and power amp)
I have 370-395v for B+ actual. That is right in line with what I wanted. I used this same PT for another build (4-12ax7's instead of 3) and the B+ is right in line with that amp.
Rectifier is solid state-4 UF diodes
I dont know what the voltage on the 5v is because it is shrink wrapped and tucked away.

Why I used diodes to drop the voltage was what I read in Merlin's book.


Another thought: I used a led for "on" status and I used a diode and 1k to power it.

[bob p]

OK, now we know a few things:

1. the voltage on your 5V winding does not matter.
2. the voltage on your HT winding is good.
3. the voltage on your 6.3 V winding is the problem.

Just for the sake of an example, let's add up the load on your 6.3V line. The transformer is spec'd to deliver 6.3V when loaded with 5A.

(2) 6V6 @ 450mA = 900mA
(3) 12AX7 @ 300mA = 900mA

Adding up the loads, you've got a total load of 1.8A. Your voltages are high because the transformer has less than half of the load that it's rated for.

Like Juan said, you have two options:
1. lower all voltages by using the 125VAC primary tap, or
2. add series resistance.

If you're happy with your B+ voltages, then you can ignore my earlier recommendation for option 1 and go with option 2.

[bob p]

> I used this same PT for another build (4-12ax7's instead of 3) and the B+ is right in line with that amp.

that makes sense. those two amps differ by a single 12AX7. all else being equal in the circuit, the individual loading of the power supply from one 12AX7 isn't very significant so I'd expect the voltages to be similar.

One thing that you might consider is looking at the 6.3V supply voltages on that other amp to make sure they're not running high.

[chunkitup]

Wow I never knew that if the load is too light the voltage would go higher than what its spec'd for. Good to know. Thank you for help Bob and the other guys.