Ampeg V4: running at half power (2 vs 4 tubes)

[boroman]

I recently did it in a 3-day live recording session because we were recording live in the same room and it was just too loud for crunchy V4 sound. We did removed outer tubes and put the ohm selector to 2 ohms (running 4ohm cab). Is this safe, let's call it, "mod" for that amp? Does it hurt the transformers? Or something else?

 

[waltdogg]

If you did it right, then yeah. My buddy's V-4 has that mod, whenever you engage it (we turned the polarity switch in a 1/2 power switch) and want to use 4 ohms you set the head to two ohms. As when you split the transformer like this, ohmage doubles. I'm not sure why.

 

[beans-on-toast]

What you did will work without causing any damage to the transformer.

The problem is, even with two tubes pulled, the amp is still very loud. The VT-40 is the smaller brother of the V4B. It has half the power of the V4B, with two 7027A output tubes. Those amps can still put out 70 clean watts.

 

[beans-on-toast]

My buddy's V-4 has that mod, whenever you engage it (we turned the polarity switch in a 1/2 power switch) and want to use 4 ohms you set the head to two ohms. As when you split the transformer like this, ohmage doubles. I'm not sure why.

You want to keep the impedance matched between the output tubes and the speaker cabinet. Think of the transformer as device that reflects the impedance of the speakers to the tubes. Remove two of the tubes, you are cutting the total impedance presented to the transformer in half. When you connect a 4 ohm speaker to the two ohm tap on the transformer, it works out that the impedance that the tubes present is matched properly to the speaker. Or close enough.

 

[boroman]

B-O-T, yep, I realized that It was not cut by 50% Still loud as hell! But the breaking point moved a bit lower so it was easier to get the right sound with not so much bass going to drum OH mics

 

[beans-on-toast]

B-O-T, yep, I realized that It was not cut by 50% Still loud as hell! But the breaking point moved a bit lower so it was easier to get the right sound with not so much bass going to drum OH mics

So how did the recording sessions with that amp go?

 

[will33]

This is something that I think can be done with most any tube amp that runs its output tubes in push/pull pairs, which is most of them. The SVT is one that doesn't as it runs in triplets (2 pairs of 3).

The part some people forget is to double the OT impedance tap.

Did this to play guitar with my Bassman135 all the time. Still Fender clean tone as loud as you could stand it, but at least the ears only hurt instead of bleed.

 

[JimmyM]

This is why a B-15 or SB-12 is so sought after for the studio.

 

[boroman]

So how did the recording sessions with that amp go?

Oh, great! I was a little worried that engineer told me "it's too loud" and the bass was going to all condenser mics. We've tried with the cab placement but ended with removing tubes. What about bias voltage? Does it change in that operation? Safe for tubes?

 

[beans-on-toast]

Oh, great!

What about bias voltage? Does it change in that operation? Safe for tubes?

A little bass leaking into the other mics isn't the end of the world. Drums doing this can be a problem.

Just to be sure, it's always a good idea to check the bias voltage at least once when you change anything related to the power tubes. You should be fine. Bias isn't this ultra precise thing that has to be a certain number. Anywhere within a safe operating range is acceptable. If the amp sounds good and it is close to where it should be, it's the right bias setting.

Of more concern is that you are wearing your set of four tubes down unevenly when you pull to of them. The best thing to do is to rotate the tubes. One time take out the middle two, the next time take out the two at the end. If you want, you can keep a record of how long you ran the amp with any pair and then you can balance it out. If you don't do this, when you have four tubes installed, eventually one pair will be working harder than the other pair. Remember tubes wear down like car tires. It is good to rotate them to keep the wear even.

 

[boroman]

yep. I know that. tubes are GE from 1975 as i remember and they still perform great. I was just wondering if bias voltage is going 2x higher? because if we have 40ma than having 50ma per tube (so 20%) is way too much... what is "less tubes-to-bias" ratio?

 

[beans-on-toast]

The bias voltage will not be two times higher.

If your tubes are matched, they are performing more or less the same. Remove two tubes and the same cathode current should be flowing through each tube.

In theory, current does not flow into the grid where the bias voltage is applied, no current flows so the voltage doesn't change. You set the voltage and the bias remains fixed as the tube operates, hence the name fixed bias. Pull out all the tubes and the bias voltage should remain the same.

 

[boroman]

Thanks. I remember one time someone told me about the SVT when you remove one tube per side (so they are 2 vs 2) the bias is rising up and it is a fixed bias amp. Maybe he got it wrong.

 

[dunwichamps]

The bias voltage will not be two times higher.

If your tubes are matched, they are performing more or less the same. Remove two tubes and the same cathode current should be flowing through each tube.

In theory, current does not flow into the grid where the bias voltage is applied, no current flows so the voltage doesn't change. You set the voltage and the bias remains fixed as the tube operates, hence the name fixed bias. Pull out all the tubes and the bias voltage should remain the same.

Well yes/no. When you drop two of the tubes out then the secondary HVAC is now seeing half the normal load on it so likely the B+ has increased but the bias voltage has remained constant so depending upon how much loading there is when all 4 tubes are at idle there could be a slight increase in the idle plate dissipation when pulling a pair.

 

[beans-on-toast]

Well yes/no....

I didn't want to complicate things by getting into power supply impedance issues. In practice the bias on the V4 is close enough when this is done.

As I said earlier, it's always a good idea to check the bias at least once if you are planning on removing two of the tubes.

 

[Jeff Scott]

ull out all the tubes and the bias voltage should remain the same.

The Bias Remains The Same - great song title..............

 

[BassmanPaul]

If you put a switch to turn off a pair of output tubes wire a 5-10KΩ 5W resistor across it to allow some current to flow through the idle tubes. It's better for the tubes than leaving them off with the heater on.

 

[waltdogg]

If you put a switch to turn off a pair of output tubes wire a 5-10KΩ 5W resistor across it to allow some current to flow through the idle tubes. It's better for the tubes than leaving them off with the heater on.

+1. This. It's so much better to be able to choose your power at any time without having to fumble with tubes. Plus, it's safer and ensures the amp will continue to run healthy for a long time.