Need help on Kenwood KR 7070

[Motto26]

Hi

This tube amp was given to me. The brand and model is Kenwood KR 7070. I searched the net and this model is no where to be found. I'm in the process of restoring this baby and I dont know where to start aside from cleaning the panel etc... it has 4 12AX7 pre-amp tubes and 4 KT88. But I'm not so sure if its really KT88 but the picture of GEC KT88 and the tube that is installed is similar. These are the things that I need help.

1. Schematic diagram
2. Will it get damage if its not properly Biased
3. How will i know if its 110 or 220 V
4. Is there really a Kenwood KR 7070 tube amp?

[Tubologic]

Originally posted by Motto26
Hi

This tube amp was given to me. The brand and model is Kenwood KR 7070. I searched the net and this model is no where to be found. I'm in the process of restoring this baby and I dont know where to start aside from cleaning the panel etc... it has 4 12AX7 pre-amp tubes and 4 KT88. But I'm not so sure if its really KT88 but the picture of GEC KT88 and the tube that is installed is similar. These are the things that I need help.

1. Schematic diagram
2. Will it get damage if its not properly Biased
3. How will i know if its 110 or 220 V
4. Is there really a Kenwood KR 7070 tube amp?

Are you sure about the brand and model Nr. ? There is a Kenwood KR 7070 model but it is a (solid state) receiver. Usually, Kenwood models starting with KRxxxx are receivers,not amplifiers (starting with KAxxxx). Early (tube) Kenwood products were often branded TRIO. Also,there are VERY FEW integrated amp's using the KT88/6550 type tubes and judging by the (relatively small) size of the OPT's there is a serious doubt that the output tubes in your amp are the right ones. Yes,they look (and probably are) like GEC KT88's. Sometimes Japanese used strange tubes in their amps,including TV horizontal sweep tubes. Who knows ? I can't be positive on this.

1. No schematic to be found coresponding to this brand/model Nr.
(except a solid-state receiver)

2. YES, incorrect bias could damage ANY amp and also very likely the output tubes by excessive plate dissipation.

3. Connect your amp first to 110V and monitor the tubes heater voltage: if it's 6.3VAC your amp is set up for 110V , if it's about 3.15VAC the amp is set up for 220V . It is recommended to use a VARIAC for this test, or disconect the transformer HV secondary to avoid damaging the amp.

4. AFAIK , the answer is NO.

To restore this amp you need to follow standard practices already described in length in this forum. The most vulnerable parts in vintage Japanese tube amp's are the coupling (paper) caps,especially the gray colored ones. Look inside the chassis and if you find these gray cap's they must ALL be replaced (believe me) BEFORE powering up the amp because they are usually leaky,especially when they warm up. NEVER TRUST OLD JAPANESE GRAY PAPER CAP's !! Electrolytic's must be checked too and reformed/replaced if necessary. Sadly, vintage Japanese tube gear suffers from sub-standard parts in comparison with U.S made gear from the same period and the restoration process is much lengthty. (more parts to replace) Drifting value and/or noisy resistors are very common too,as well as defective pot's (noisy,very poor tracking or open tracks) which could be the greatest challenge to fix. Luckily, KENWOOD was a better quality brand than (say) LAFAYETTE or NATIONAL and hopefully all the remarks above will not apply to your amp. Good luck with your project.

[Bandersnatch]

hey-Hey!!!,
I looked at this thread 'cause I had one of the SS KR7070's. It looked nothing like yours of course... As you've been warned, don't trust the passive components, 'specially the coupling caps. Also, the tube with white on top is bad, no saving it.
cheers,
Douglas

[Motto26]

I thought of that as well. its oxidized and a gas might have its way into the tube but I couldn't find any cracks on it. Now I'm having second thoughts of restoring this amp as it would cost me more time and money. I'm now planning to build a tube pre-amp with a solid state main amp. Would you think this is a good idea?

[Klimon]

Would you think this is a good idea?

No.You've got the most expensive parts (power and output transformers + output tubes if they are in good working order) of a KT88 PP on hand + neatly installed on a chassis together with tube sockets and room plenty to install new caps and resistors. What more could you ask for??

The circuit will more than probably suck, so search the net for a proven diy kt88 PP circuit, design a power supply for the available x-former and lay some wires... finito!

This imo is a more logical and better sounding solution

Simon

[Motto26]

Thanks Simon. Now I'm having second thoughts of breaking apart this amp for scrap. But the power tube doesn't really indicates that its a KT88 Tube. It just says GEC made in England.