couple of questions re: my REAL 1974 ('67) 18w

[sreiter]

Does anyone know what each "valve"(tube) position does?

Is V1 only for the non tremolo side, like on a deluxe reverb, or is it cascade to V2? Trying to figure out where I should put my mullard vs bugle boy, ect. before i try every combo I can aka what does conventional wisdom say

my amp is "220v" or whatever the actual wall aka "mains" was in the 60's .. (213-253 per the internets, but folks say in some places it ran 240v-250v consistently).

My the IT tap is set 250 (240/250?). Ive run my amp a few time at 220 just to try to save not run so hot. Am I doing more damage by running so low voltage?

[Bieworm]

Normally the classic 1974 tubes are as follows:
V1: preamp normal channel
V2: phase inverter (LTP)
V3: preamp and oscillator tremolo channel

A lower input voltage translates to lower plate and screen voltages, which is easier on the tubes. But the lower the voltage, the lower the headroom, so that’s up to you. Ideally you check the circuit’s voltages and compare them to the specs. FWIW the 1974 is known for running very hot. This has to do with the bias setting on those. There’s no problem to run cathode biased amps fairly hot, but if you want to expand the lifespan of the EL84 it’s better to set the bias around 80-85% dissipation

[sreiter]

thanks!!

[sreiter]

Normally the classic 1974 tubes are as follows:
V1: preamp normal channel
V2: phase inverter (LTP)
V3: preamp and oscillator tremolo channel

A lower input voltage translates to lower plate and screen voltages, which is easier on the tubes.

this makes perfect sense to me, but i brought my amp to a shop in Nashville, and the owner makes his own amps and has a few big names use them.

He said just the opposite, and it had my head spinning. His said if i lowered the voltage, it would be cooler for the tubes. I dont get it.

Does that make any kind of sense?

[Daviedawg]

The outcome is an inverse. The design of the transformer with selectable taps is so that you can select a transformer tap for the expected wall voltage. It does not change the wall voltage, it changes the secondary voltage. So if you select a higher primary you get a lowering of the secondary to try to match the design secondary.

This can be used to manipulate the secondary output of the transformer and subsequent voltages. I use the 250v on my Watkins Scout for just that purpose.

Dd

[JMPGuitars]

To put it in simpler terms: higher voltage than expected from the wall will result in higher voltage on the tubes. Higher voltage on the tubes (plates) will give you higher clean headroom. That means lower voltage on the tubes (plates) will give you distortion sooner.

It's also important to consider what the amp circuit and power transformer were designed for. If it was designed for 220, and that's about the wall voltage the power transformer sees, then you're amp should behave as was intended. If the power transformer has labeled taps, for example 220 and 240, and you match the wall voltage to the labeled tap on the power transformer, than nothing should change. OTOH, if your wall voltage is 240, and you tap the 220, that will give you higher headroom since you're throwing more voltage at the power transformer than expected.

[sreiter]

The outcome is an inverse. The design of the transformer with selectable taps is so that you can select a transformer tap for the expected wall voltage. It does not change the wall voltage, it changes the secondary voltage. So if you select a higher primary you get a lowering of the secondary to try to match the design secondary.

This can be used to manipulate the secondary output of the transformer and subsequent voltages. I use the 250v on my Watkins Scout for just that purpose.

Dd

thanks

[sreiter]

To put it in simpler terms: higher voltage than expected from the wall will result in higher voltage on the tubes. Higher voltage on the tubes (plates) will give you higher clean headroom. That means lower voltage on the tubes (plates) will give you distortion sooner.

It's also important to consider what the amp circuit and power transformer were designed for. If it was designed for 220, and that's about the wall voltage the power transformer sees, then you're amp should behave as was intended. If the power transformer has labeled taps, for example 220 and 240, and you match the wall voltage to the labeled tap on the power transformer, than nothing should change. OTOH, if your wall voltage is 240, and you tap the 220, that will give you higher headroom since you're throwing more voltage at the power transformer than expected.

thanks

I'm using the amp in the USA so i need a step up/down transformer, but because the wall voltage swings wildly, I use a variac in front of the step up, so theoretically I can send anything the trans can see, whichever jumper setting.

thanks

[JMPGuitars]

I figured you had a setup like that. Have you measure the output voltage of the step transformer? If it has regulated output, the variac may not be necessary.

[sreiter]

sorry for the late reply

I have measured the output of my step up/down trans and from the wall I'm getting 240. I'm in the nashville area and current fluctuates wildy. I'm running from the wall to variac to keep a semi stable current, to a surge protector that I have my pedal board, '65 BFDR, and step up plugged into, then the Marshall.

My BFDR is biased for 120v, so i'm running the variac anyway.
I stopped taking the 18w out of the house, so its ok running indirectly off the variac.

Biggest issue is i bought a Brown Box for 300.00+ thinking it would regulate the voltage and its not needed.