Orange AD200B Mk II Bass Amp

[TDS]

I have one of these bass amps on the bench. I suspect that the resistor values have been changed around the 12AT7 LTP PI area.

The values currently appear to be Fender AB763 values:

Plates - 47K
Cathodes - 470R
Tail - 22k
bottom - 100R
feedback - 1K2
Input cap 0.022uF
Output caps 0.1uF

Can anybody confirm what the correct PI and feedback resistor values should be?

TIA

[Enzo]

I don't know the amp, but maybe they converted from 12AX7? Look at the schemo for the Orange Hustler at Schematic Heaven for an example.

[TDS]

Enzo, The information I have found so far indicates that the 12AX7 preamp tube, the 12AT7 PI tube and the quad of 6550's are the correct tubes for this amp. Apart from some pictures and brief information I found there don't seem to be any schematics around anywhere where I can confirm the PI resistor values. I'm hoping that somebody has been close enough to one of these amps to know what the resistor values should be.

Heres Hoping

[Enzo]

Well, you could contact Orange. Worst outcome is they tell you "no."

[Bernardduur]

I have the schematics of the Mark I and the Mark III when people are interested!

[TDS]

I would be very grateful for copies of both schematics if possible.

[Bernardduur]

Here are the schematics!

LINK

Please, comment on them. The owner of the Mark I finds it pretty sub-bassy and needs more treble. As you can see it also lacks a NFB.........

[TDS]

Thanks, I've been looking for this info for quite a while. I'll comment further when I've looked over them in detail.

[Bernardduur]

I have both of em rigth in front of me so if persons have questions I would love to answer em!

[TDS]

A few questions if it is possible for you to answer:

MK1 Amp

1. What are the V4, V3, V2 and V1 voltages.

2. What are the AC output voltages from the two HT windings to the bridge rectifiers?

MKIII Amp

1. What are the V4, V3, V2 and V1 voltages.

2. What are the AC output voltages from the two HT windings to the bridge rectifiers?

3. The secondary winding of the OPT shows only one winding for both 4 and 8 ohm speakers. Is this correct?

4. What type of diodes are D3 and D4 in the poweramp. Is it possible that they are LED's as was used in the Trace Elliot V8 Bass Amp to indicate blown fuses?

Any answers will be appreciated.

[Bernardduur]

I'll answer em later!

Right now I'm trying to bias the MKIII version. I'm used to most bias versions but as I want my 'clients' to bias em also I usually install the 1 ohm resistors in my builds.

When I do this in the MKIII the total resistance varies between 2.4 and 4.2 ohm because of the fuses. I decided to leave the fuses out of the circuit and wire the 1 ohm resistors directly to the ground.

Now when I bias this little amp I can't get higher then 21.2 mA which means approx 13.1W per tube........ My calculations showed that I needed to go to (approx 70% of 46W = 32.2W).

I hate this little POC


Guess I need to redesign the bias section. Now when I look back at the stock bias setting (with a set of EHX 6550) the bias was set around 13 mA.........

Stupid amp

[Bernardduur]

A few questions if it is possible for you to answer:

MK1 Amp

1. What are the V4, V3, V2 and V1 voltages.

2. What are the AC output voltages from the two HT windings to the bridge rectifiers?

MKIII Amp

1. What are the V4, V3, V2 and V1 voltages.

2. What are the AC output voltages from the two HT windings to the bridge rectifiers?

3. The secondary winding of the OPT shows only one winding for both 4 and 8 ohm speakers. Is this correct?

4. What type of diodes are D3 and D4 in the poweramp. Is it possible that they are LED's as was used in the Trace Elliot V8 Bass Amp to indicate blown fuses?

Any answers will be appreciated.

As for the MKIII

1. I'll check back when I biased her correctly!

2. 474 VAC and 267 VAC

3. Woops; I'll correct that!

4. They are 3mm LED's. They'll light when a fuse is down

[TDS]

I'll answer em later!

Right now I'm trying to bias the MKIII version. I'm used to most bias versions but as I want my 'clients' to bias em also I usually install the 1 ohm resistors in my builds.

When I do this in the MKIII the total resistance varies between 2.4 and 4.2 ohm because of the fuses. I decided to leave the fuses out of the circuit and wire the 1 ohm resistors directly to the ground.

Now when I bias this little amp I can't get higher then 21.2 mA which means approx 13.1W per tube........ My calculations showed that I needed to go to (approx 70% of 46W = 32.2W).

I hate this little POC


Guess I need to redesign the bias section. Now when I look back at the stock bias setting (with a set of EHX 6550) the bias was set around 13 mA.........

Stupid amp

I am also a fan of using 1 ohm resistors in the cathode path of the output valves. Safe and simple and accurate measurement, as long as you remember to subtract the screen grid current from the cathode current

Is it not possible to arrange the tapping point for the 1 ohm resistors so that the measurement is not being made across the fuses as well as the resistors?

I guess that the reason the bias supply lacks the range you are looking for is possibly because Orange apparently recommend a current range of between 18mA (Cool) and 22mA (Hot) more like Ice Cold and Luke Warm!

What is the bias voltage range that you are getting at he moment?

A small side issue, looking at your calculation, is the max dissipation of your 6550's 46 watts? The highest reference I can see for the 6550EH is 42 watts and that is higher than most other 6550's at 35 Watts.

It should be simple enough to increase the current by pulling down the bias range by adding a resistor in parallel across R46 to give you the adjustment needed. I usually aim for around 50% dissipation, but that is only my preference as a reasonable balance between valve life and tone.

[Bernardduur]

I am also a fan of using 1 ohm resistors in the cathode path of the output valves. Safe and simple and accurate measurement, as long as you remember to subtract the screen grid current from the cathode current

Is it not possible to arrange the tapping point for the 1 ohm resistors so that the measurement is not being made across the fuses as well as the resistors?

I guess that the reason the bias supply lacks the range you are looking for is possibly because Orange apparently recommend a current range of between 18mA (Cool) and 22mA (Hot) more like Ice Cold and Luke Warm!

What is the bias voltage range that you are getting at he moment?

A small side issue, looking at your calculation, is the max dissipation of your 6550's 46 watts? The highest reference I can see for the 6550EH is 42 watts and that is higher than most other 6550's at 35 Watts.

It should be simple enough to increase the current by pulling down the bias range by adding a resistor in parallel across R46 to give you the adjustment needed. I usually aim for around 50% dissipation, but that is only my preference as a reasonable balance between valve life and tone.

Datasheet for the 6550 of JJ

I usually listen per tube amp what suits better....... usually for bass amps I get to 50% - 60%........ 70% for guitar amps!

[Bernardduur]

Oh, and I 'fixed' the bias by adding another 68k parallel to the onboard 68k.