Can Ref. 110 change tube from 6550 to KT120?

[racewong]

Hi all ARC users, I am also a ower of Ref 110 which is using original Tube 6550.

Here is my hi-fi setup.

FULL set ARC CD8 + REF5 + REF110 + MAGNEPAN 1.7.

May I know the setting of ref 110 if I want to change the power tube from 6550 to KT120?

Or just plug-in the KT120 in ref 110 without any setting?

Thanks for your help.

 

[LL21]

Beautiful setup! I am pretty sure most of the later CJs and ARCs can use KT120 without a problem. As for your question about re-setting something, you may wish to check with ARC before you try. I think with CJ they are interchangable without changing settings. Also look here for 'Microstrip' who i think has done this on his ARC equipment in the past.

 

[dafos]

Local dealer has retubed several ref 110's with kt 120 with no problems. For precaution, I would back off on the bias setting on your current 6550's to about 55ma before installing the kt 120.

 

[microstrip]

Hi all ARC users, I am also a ower of Ref 110 which is using original Tube 6550.

Here is my hi-fi setup.

FULL set ARC CD8 + REF5 + REF110 + MAGNEPAN 1.7.

May I know the setting of ref 110 if I want to change the power tube from 6550 to KT120?

Or just plug-in the KT120 in ref 110 without any setting?

Thanks for your help.

You can change the tubes from 6550 to KT120 in the REF110 - you just have to re-bias the unit. Remember the tubes must be well matched - as the unit uses 8 tubes I always try to get 9 or 10 to have immediate spares just in case one goes bad or goes out of range of bias. This is particularly important if you are not sourcing your tubes from ARC, as most sellers do not carry burn-in in tubes.

You will notice a considerable improvement when you change the power tubes to KT120's.

 

[racewong]

Thank you microstrip, May I know how to setup the bias for my Ref110?

Becoz I am a newbie in the tube-world! ^^

 

[dafos]

The biasing procedure is clearly outlined in the ref 110 manual. You will need a digital multitester and a plastic screwdriver that I believe is supplied with the amp. It's a pretty straightforward procedure, but it does involve getting your fingers close to the output tubes.

 

[DonH50]

I think he is wanting to know if the bias current changes. It does not need to change going from 6550's to KT120's. The KT120 can handle higher current, but unless the rest of the circuit (including output transformer) is designed to handle the extra current I would not increase the bias current. Check with ARC to be sure. In any case, just swapping tubes and setting at the same bias level as the 6550's should be fine.

HTH - Don

p.s. Another Magnepan owner, great!

 

[rblnr]

ARC told me that a circuit change is needed for the 210s anyway.

 

[DonH50]

To just swap the tubes, or to re-bias the KT120's for optimal operation?

 

[microstrip]

I think he is wanting to know if the bias current changes. It does not need to change going from 6550's to KT120's. The KT120 can handle higher current, but unless the rest of the circuit (including output transformer) is designed to handle the extra current I would not increase the bias current. Check with ARC to be sure. In any case, just swapping tubes and setting at the same bias level as the 6550's should be fine.

HTH - Don

p.s. Another Magnepan owner, great!

Don,

You have to re-bias the amplifier when you change from 6550 to KT120. The bias current should be kept the same - it is checked by measuring the voltage across the cathode resistors at some test points

I have built a jig to test and match 6550 and KT120s at the operational voltage and current used by ARC amplifiers. Although it varies with the lot and tube, on average I found the KT120 needs about more 5V negative grid potential than the 6550C to get the ARC specified current.

Besides filament current, there is another parameter that must checked when replacing the 6550C with the KT120 - the maximum control grid voltage. It is -350V for the 6550C and -200V for the KT120. Designs using DC cathode coupling from the driver to the control grid, such as used in some McIntosh's and ARC's, can have problems.

 

[dafos]

To just swap the tubes, or to re-bias the KT120's for optimal operation?

To swap tubes, a circuit modification kit is provided by ARC for the ref 210 and 610.

 

[DonH50]

Don,

You have to re-bias the amplifier when you change from 6550 to KT120. The bias current should be kept the same - it is checked by measuring the voltage across the cathode resistors at some test points

I have built a jig to test and match 6550 and KT120s at the operational voltage and current used by ARC amplifiers. Although it varies with the lot and tube, on average I found the KT120 needs about more 5V negative grid potential than the 6550C to get the ARC specified current.

Besides filament current, there is another parameter that must checked when replacing the 6550C with the KT120 - the maximum control grid voltage. It is -350V for the 6550C and -200V for the KT120. Designs using DC cathode coupling from the driver to the control grid, such as used in some McIntosh's and ARC's, can have problems.

Ah, thanks. I knew they would need to be re-biased, as in any tube swap (same model or not), and that the KT120's would run with the same current as the 6550's. I have had a number of tube amps, had friends with them, and worked on hundreds. I was fortunate to have access to a nice dynamic tube tester back when I was running tubes in my system -- it helped match more closely than a single-point check.

I had guessed there was enough voltage range to adjust so bias current remained the same; I had forgotten ARC DC-coupled drivers to outputs. That would explain the circuit changes. I have a vague memory of tweaking the regulator and zener string in a D79 to use KT120's (or KT88's -- long time ago) but had forgotten the details.

I have a vague memory of setting up a McIntosh with KT88's (I think) and it took some work... Unlike ARC, the Mc factory techs did not help IIRC and I seem to recall having to work out the schematic on my own. I seem to recall having to tweak some other parameters like feedback networks...

Thanks - Don

p.s. Curious -- back then, one of the reasons was to increase power output by adjusting the bias up to take advantage of the KTxx's greater dissipation capability -- does anyone do that now?

 

[Lee]

I tried to add KT120s to my VT-100 but the factory has decided there are a few issues.

1. The tube is a bit heavy which may put too much pressure on the tube socket.

2. Needs a larger transformer for more current to the plate.

3. Some issue around heat dissipation so I need a fan.