Orange Thunderverb 200 bias current

[mikeydee77]

I have the above amp on my bench (manufactured 2011) and it just had a problem with the reverb cable but having fixed that the other issue is that the owner thinks the output is a little low compared with the 50w version.

I agree that it does seem a little low although when maxed out and driven with a higher than normal signal (say 1.5 volt rms on the input) I can just about get close to 200w.

I checked the bias current and all tubes are about 20mA 650v plate voltage. I thought that seemed a little on the low side (expecting 30-35mA) so I decided to adjust up to 30mA to see if that improved matters but what I find is that the bias pot is already maxed out and can not be increased further.

Would like to hear from anyone that has set the bias on one of these or has the schematic or other views like this is normal for this amp etc.

Also amp sounds great and there is no crossover distortion on the output.

I don't have a schematic and Orange don't seem willing to provide.

[g1]

If a 50W version seems louder, and you need a 1.5VRMS signal at the input to get full power, there is something wrong with the amp.
If you plug guitar into the FX return, how does it sound?
Maybe try another preamp into the FX return, or the FX send into another power amp.
I don't think low bias would result in you needing to provide a 1.5V signal to the preamp input. But some other problem in the power amp (low screen voltage etc.) could result in low bias.

[The Dude]

Holy shi*&, 650V on the plates! Are you sure the battery isn't dead in your meter?

[Jazz P Bass]

More than likely they are using a clamped down screen voltage, ala Univox.

[mikeydee77]

Thanks for the suggestions.

Are you surprised by 650v on the plate? I think that is ok based on the warning printed on the PCB IN CAPITALS about there being up to 700v present. I am not sure about 6550 but I know kt88 have a max plate voltage of 800v so sounds sensible.

The screen resistors are the typical ceramic 1k jobs they measure ok though I haven't checked the voltage on those just yet as I was hoping a schematic would be somewhere in the mef cloud.

These amps have an attenuator control which lowers the output power somehow. The other power shaping circuits that I have seen manipulate the high voltage and bias proportionately so given that I can't adjust the bias very much I wanted to have a look at that part of the circuit to see if that could be a cause.

Some tube measurements
Plate 650v
Screen 375v
Grid -44
Heaters 6.3
Cathode 0v

Thanks for reading and a happy new year

[mikeydee77]

If a 50W version seems louder, and you need a 1.5VRMS signal at the input to get full power, there is something wrong with the amp.
If you plug guitar into the FX return, how does it sound?
Maybe try another preamp into the FX return, or the FX send into another power amp.
I don't think low bias would result in you needing to provide a 1.5V signal to the preamp input. But some other problem in the power amp (low screen voltage etc.) could result in low bias.

Re-measuring the amp more accurately I can get 180w into 8 ohm with a 750mV rms signal. Going straight into the return of the fx loop is the same as the volume flat out and the gain about 4-5.

See my other post with measurements but screen voltage is there at 375v. I am assuming that is ok.

[Jazz P Bass]

Re-measuring the amp more accurately I can get 180w into 8 ohm with a 750mV rms signal. Going straight into the return of the fx loop is the same as the volume flat out and the gain about 4-5.

See my other post with measurements but screen voltage is there at 375v. I am assuming that is ok.

Well I guess an 'active' pickup might put out 750mvs.

As assumed, the screens are held way low, like the Univox 650/ 700 volt amps.

My question would be, what is the amps output voltage with a 'normal' 100 mv input signal?
Just at the onset of clipping.

[mikeydee77]

Oh dear back to the world of post xmas real work . . hence the gap on fun stuff like this amp.

As suggested by Jazz P Bass I applied a typical 100mV rms signal to the amp.

Retaining a clean output I can get 35v rms into 8 ohms (approx 160w)
Absolutely flat out I can get 45v severely clipped into 8 ohms (approx 225w)
At this point coming out of the send loop is 500mV

So maybe this amp is ok it is just that all the action happens at the end of the dial.

I have drawn out a partial little schematic below

[Jazz P Bass]

160 watts is pretty healthy for four output tubes.

Quote"the other issue is that the owner thinks the output is a little low compared with the 50w version"

Same speakers?
Clean or distorted?

[GainFreak]

I checked the bias current and all tubes are about 20mA 650v plate voltage. I thought that seemed a little on the low side (expecting 30-35mA) so I decided to adjust up to 30mA to see if that improved matters but what I find is that the bias pot is already maxed out and can not be increased further.

Same thing here - 20mA per tube (615V plate, 350V screen).
Since all 6550 data sheets I've seen recommend 50-55mA per tube (at 600V anode and 300V screen, no signal, which is like ) it's kind of strange to see those amps biased so cold unless they had something special in mind which I currently don't understand.
The bias pot maxed at 40mA plate (now 600V anode) which is ~50% dissipation.
By the way the hand drawn schematic above is not correct. R1 and R5 are not 1 Ohm but 100k resistors and obviously are not there for reading the current.

[g1]

it's kind of strange to see those amps biased so cold unless they had something special in mind which I currently don't understand.

Does it sound bad with the bias so cold? If not, what they had in mind is tube life & reliability.

[The Dude]

I think there's some confusion here with bias as it relates to output. There is little correlation between the two. Bias has an effect on signal at low volumes where there is not enough grid voltage for tube conduction. It will have little effect on maximum power output.

[GainFreak]

Does it sound bad with the bias so cold? If not, what they had in mind is tube life & reliability.

I don't know and honestly I don't care - it's not my type of sound and amp. The owner just said "Bias is cold, make it hotter". Who am I to argue? But he is right. By all standards and datasheets the bias is cold.

[GainFreak]

I have one of these in front of me for retubing and I'm somewhat confused. What should be the 1 Ohm resistors (R1 and R5 according to the schematic above) have the color code for 100 kOhm ones (brown-black-yellow(should be gold)-gold). However they measure as shorted (zero Ohms). Currently bias voltage is -47V.
Also except for the lower output what's the deal with 1+3 and 2+4 tubes?

[Enzo]

Hi GF

This thread is like 6 years old and about a different unit. It can be confusing to look at voltage readings and other observations and not realize we have two amps. I suggest starting a new thread for your amp.

Also except for the lower output what's the deal with 1+3 and 2+4 tubes?

I am not sure what this refers to.