Will KT88 increase output power in Copland CTA-402

[Bill Kilgore]

Hi everyone,

First of all, hello ! this is my 1st post in this forum, although I've been following some threads for a while. Congratulations, it's a great place to learn and have fun !

Well, coming back to my question:

I have a Copland CTA-402 integrated amp, with EL34 tubes, giving 35 W power output.
I am using it with 86 dB speakers. Overall, I'm very happy, but with some recordings (apparently recorded at low-level) you feel that you would need some extra output from the amplifier...

My question is, would KT88 (instead of the regular EL34) help obtaining some extra Watts in terms of output power of my amp ?

I've been told that I can use the KT88 without problems (only bias adjusting).
Additionally, I've read that the circuit and transformers in that amplifier were "over-specified".

I also find an article saying that:
"...four output valves, apparently pentodes. They supposedly run close to ultra-linear Class A operation, churning out 35 watts per channel ... Copland says the CTA 402 will output over 50wpc without running as hot as typical Class A designs."
http://sites.thestar.com.my/audio/st...udiofile/05cop

However, even if the circuits + transformers allow using KT88, I don't know if this just translates into a different "character" in terms of sound, or if it really adds extra output power.

What do you think ?

Thank you very much in advance !
Regards

[boywonder]

EL34 and KT88 pinouts are the same, but the optimal operating conditions are different for each tube. The power output is determined by the anode voltage of the output tube and the bias current, so just swapping out the tubes won't give you any more power without upping the B+ voltage or the bias current or both.

Do you happen to know if the amp is fixed bias or cathode bias? Do you know what the B+ voltage is?

If the amp happens to be fixed bias (making it easy to increase the bias current), the next question will be can the PS transformer handle it?

The linked review appears to have a little hyperbole; You won't get 35W class A out of a pair of UL wired EL34's............even "near" class A is a stretch.

A UL wired EL34 likes a load of around 6K, and the KT88 likes a load of around 3-4K or so. That means output transformers optimized for EL34 won't be optimized for KT88.

As a sweeping generalization, the KT88 gives nice bass and the EL34 gives great mids/vocals.

Have you considered more efficient speakers? 86db is quite low. Moving up to say 90db sensitivity will more than double the volume with the same amp power.

[Bill Kilgore]

Thanks a lot Boywander,

I'm trying to get the answers to your questions (fixed bias or not, B+ voltage, etc).

I wouldn't like to change the speakers. I actually just purchased them because I fell in love with their sound... :-) and generally speaking I am pretty satisfied with the amp-speakers combo. The speakers have a very flat impedance curve and they are a relatively easy load for the amp.
It's only in a few recordings that you feel like a few extra watts would be required, that's why I'm wondering if I could increase the output somehow.

I'll check and try to obtain some additional info on your questions.
Thanks a lot for such a detailed reply, I am "starting" and I appreciate your guidance.

Regards.

[boywonder]

Bill: According to the review in your link, the amp is fixed bias with an adjusting pot for each output tube, which makes increasing the bias current very easy, assuming that the power transformer is up to the task.

A quick search on the web also revealed that the stock current is 45ma/tube for the EL34's, which is a voltage reading of 400mv at the various test points. See the link. Make sure the amp is good and warmed up before checking/adjusting. Don't power up the amp without the speakers (or an 8 ohm R) connected.

How to replace the EL-34 tubes in your Copland CTA-402 and set the bias

Keep the following two things in mind if you want to play with the bias adjustments:

Adjusting one affects the others so when changing, you need to go around a few times to get the bias for each tube the same.

Be mindful of the power dissipation capabilities of the output tubes, which is the B+ voltage X bias current. EL34s are rated at 24W, and KT88s are rated at 42W. This is not output power, it's the max power the tube can dissipate as heat before bad things happen like red plates, melting grids, etc. When the internal tube structures melt/deform, they can short out and damage other stuff, like your output transformers, speakers, etc. Increasing power dissipation also shortens tube life. All that being said, it's not too uncommon to find amps that exceed these ratings right out of the box.

Do you have a voltmeter? Are you comfortable around high voltages? Setting the bias using the procedure in the link above involves measuring the drop across the cathode resistor which is low voltage, so no worries there. Pin 3 to ground of the output tubes will give you the anode/B+ voltage. This is probably around 400VDC or so. If you have a voltmeter and want to check this, keep one hand in your pocket when measuring. Clip leads on the voltmeter make this measurement very safe, since it can be done without sticking your hands in a live amp.

I (and lots of others here) can walk you through some basic measurements to determine if KT88s can live happily in your amp if you have a voltmeter and are comfortable taking a few measurements. Again, an $8 set of clip leads are $$ well spent.

The other thing to check is how much adjustment you have in the bias adjust pots, since the KT88's will require a different bias setting than EL34's. Following the link instructions above, if you turn one tube's pot fully counterclockwise and measure the voltage from pin 5 of the EL34 to ground, that would answer that question. This can be done without risk to the tubes if you remove the tubes before taking the measurement. For EL34's the pin 5 to ground value is around -36V or so, and for the KT88s it needs to be -55V or so. With the amp powered down, remove the output tubes, turn the amp on, and turn a bias pot fully counterclockwise, and measure the DC voltage to ground from pin 5.

[Bill Kilgore]

Again, thanks a lot, I found your reply really useful.

Unfortunately I don't have the schematics for the CTA-402. I managed to find the schematics for the previous generation (CTA-401). Apparently they are quite similar, although the CTA-402 seems to have improved circuts + increased output power (from 30W for the 401 to 35 W for the 402)

I don't feel that comfortable with doing all that measurements, but I have a good friend with experience in such kind of tweaks that can help me.

I attach the diagram of the CTA-401 in case it can help...



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[boywonder]

Bill: If the CTA-402 is similar to the attached schematic, it will be a stretch to run KT88's without other mods. Two things that are immediately obvious: The B+ voltage is 370V (low for KT88), and the bias tap on the power transformer is -50V, which is not enough negative voltage for KT88's. You typically need a bias winding of about -70 to -80 Volts for KT88's.

The values of R44-R47 could possibly be changed to up the B+ voltage to the output tubes if the transformer secondary voltage is high enough but then you are talking soldering iron/parts replacement.

[Hi-Q]

Originally Posted by Bill Kilgore

I'm very happy, but with some recordings (apparently recorded at low-level) you feel that you would need some extra output from the amplifier...

Think this may be your answer Bill, you may need to provide more drive to the amplifier. The output power is probably there but not the overall gain of the amplifier for very low input signals. Can you borrow a pre-amp from someone just to see if that helps?

Les

[Bill Kilgore]

Originally Posted by boywonder

Bill: If the CTA-402 is similar to the attached schematic, it will be a stretch to run KT88's without other mods. Two things that are immediately obvious: The B+ voltage is 370V (low for KT88), and the bias tap on the power transformer is -50V, which is not enough negative voltage for KT88's. You typically need a bias winding of about -70 to -80 Volts for KT88's.

The values of R44-R47 could possibly be changed to up the B+ voltage to the output tubes if the transformer secondary voltage is high enough but then you are talking soldering iron/parts replacement.

Thanks again ! I'll try to obtain the true 402 diagram from my dealer in order to confirm...

[Bill Kilgore]

Originally Posted by Hi-Q

Think this may be your answer Bill, you may need to provide more drive to the amplifier. The output power is probably there but not the overall gain of the amplifier for very low input signals. Can you borrow a pre-amp from someone just to see if that helps?

Les

Hi-Q, thanks a lot for your suggestion.

To be honest, I never had a preamp, so I'm not really familiar with the way I could connect that to my integrated amplifier.

By the way, I wonder if I actually can combine them, since my integrated already has a pre-amp tube stage.

Here are some pics, in case they help you confirming the feasability of combining it with a separated pre-amp.

THANK YOU GUYS FOR YOUR HELP & GUIDANCE !!




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[Bill Kilgore]

In order to add some extra info to my last post, my integrated amp has a 4-tube preamp section (2 x 12AU7 + 2 x 12AX7), and in terms of connections, it has RCA's for phono, tuner, CD, aux1, aux2, tape-in and tape-out.