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Marantz 7 and 8B / Homade variac

uenjoymyself

uenjoymyself

Surrender to the flow
Hello friends,
Today's acquisitions are a Marantz Stereo console 7 pre and a Marantz 8B amplifier. Both in beautiful cosmetic condition and according to the original owner/seller in great working condition last time they were used. (two years ago) I want to play it safe and power up with a variac without lugging them the 45 min drive to my tech friends home. Are home made variacs dependable enough to attempt power up to these vintage jewels or am I safer just making the trip to a professional with a "real" variac?

Does anyone have a link on a DIY variac unit that is reliably safe?

Thanks to all!
Tube newbie
 

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try searching out dim bulb maybe this or diy forum. i have never used one but many around this site do
 
A variac would be fine , but it is the person with his hand on it that makes all the differance . If you don't trust yourself have your tech do it .
 
how is the variac made? If its using a lamp dimmer, thats no good. If its using light bulbs of various wattages, that'll do fine.
 
Nice score! Hope you can add some details of how you came to get these.

Actually, you should try your local CL to see if there are any variacs or powerstats (both tradenames) that might be available. Old lighting control systems are also based on variacs so that might be another source. Also, surplus electrical or electronics stores will likely have a few too. A commercial variac in the 5 amp range should be plenty adequate for your needs, 2 wire or three wire would work since the Marantz gear has no grounded power cord.

A homemade variac isn't easy because of the need to find a way to reliably vary the voltage - its not easy to wind any type of autotransformer and trying to deal with the current loads is not safe to do without knowing what you're doing.

A dim bulb tester will prevent any big current draw and will help in that regard if a variac isn't available. Both would be ideal because then you can bring the voltage up slowly and limit the current for safety.

BTW, the cabinet doesn't appear to be a Marantz cabinet, at least not one I've seen for the 7 or any typical receiver. The 7 didn't have a lot of heat generation and the tubes are out back so the regular cabinet did not have vents. But at least it provides some level of protection to it.
 
Congrats on getting those!

I would use a commercial VARIAC (a Powerstat branded unit in my case). I used the dim bulb for years, and it works, but a VARIAC is ideal. If you can get one with an ammeter to measure current draw that would be even better. If not, you can plug one of those kill-a-watt devices on the VARIAC and plug the 7 or 8B to it, it will measure current draw on it's digital display.

In any case, before using the VARIAC to power the units, I would replace the fuse in them with a fast acting type, a slightly lower rating is better (2A vs 2.5A for example). This will allow for a little extra safety if there's short in one of the units.
 
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This kind of stuff warrants some serious thought. Do you want to keep them original, or do you want to use them? Either way, I'd say they deserve the service of an experienced trustworthy tech. Make sure to get the tubes back if they are original or valuable, even if they don't test good.
 
Yep, I prefer to preserve as much originality as possible, even if the unit will be a daily driver.
Here are a couple of caps I restuffed for the 8B I currently use...
 

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Also make sure to have a load (Speakers or a 16 ohm 25W + Resistor) hooked up when test in or you risk the Output Transformers!

As mentioned if you could hook up the light bulb in series with the variac that would be best as that will tell you if you have a bad cap or short saving burning out the PS Transformer.


Randy
 
Great suggestions! :thmbsp:
I would also check the fuses on the units to see if they are intact. A blown fuse would be a sign of something going wrong in the past. OP says units were working two years ago, so the fuses should be good.


Shadowdog said:
Also make sure to have a load (Speakers or a 16 ohm 25W + Resistor) hooked up when test in or you risk the Output Transformers!

As mentioned if you could hook up the light bulb in series with the variac that would be best as that will tell you if you have a bad cap or short saving burning out the PS Transformer.


Randy
Click to expand...
 
To the OP:
What serial number range is the Model 7 in?
Mine is in the 102xx range with the green painted chassis. Serial numbers start at 10000.
 
crooner said:
To the OP:
What serial number range is the Model 7 in?
Mine is in the 102xx range with the green painted chassis. Serial numbers start at 10000.
Click to expand...

Just checking in. Thanks for the comments! Ill be back to report more as time allows. The serial # is 17xxx
 
uenjoymyself said:
Just checking in. Thanks for the comments! Ill be back to report more as time allows. The serial # is 17xxx
Click to expand...

Your unit is from 1962 and has the more desirable Clarostat volume and balance controls. These have openings that allow them to be cleaned with DeOxit if needed.

Starting at serial number 18000, the factory started using pots made by Cosmos which are sealed.
 
uenjoymyself said:
Hello friends,
Today's acquisitions are a Marantz Stereo console 7 pre and a Marantz 8B amplifier. Both in beautiful cosmetic condition and according to the original owner/seller in great working condition last time they were used. (two years ago) I want to play it safe and power up with a variac without lugging them the 45 min drive to my tech friends home. Are home made variacs dependable enough to attempt power up to these vintage jewels or am I safer just making the trip to a professional with a "real" variac?

Does anyone have a link on a DIY variac unit that is reliably safe?

Thanks to all!
Tube newbie
Click to expand...

Me........I'd drive them the 45 min to your tech but if you do go diy, you have been given some great advice.

Good luck. I'm listening to a pair of those as I type.
 
onemug said:
Me........I'd drive them the 45 min to your tech

Good luck. I'm listening to a pair of those as I type.
Click to expand...

Did that and found that the 8B was in wonderful working condition! Tested within specs for distortion, freq.response and noise. Output 30 watts both channels and bias test all four tubes exact. (I hope I have that right Im no tech I just stand back and wait for the :thmbsp: anyway my tech says its a very respectable unit)

Now the bad news. The 7 powered up and I was able to listen for a few minutes, everything was working... then..... died. No smoke no flashes or surges just worked and then didn't. This is what the P.O. had indicated so its scheduled to go under the knife this coming week.

NEXT QUESTION: I want to replace at least the power tubes in the 8B. Im sure this question is right up there with which interconnect to use as these things are subjective to user preference BUT... Id like a nice matching set that closely match the original equipment or better without spending a gagillion dollars. Without too much effort I have located 2 PAIR of new matched mallard reissue EL34's for $48.00 a pair. so about $100. Or, a quad set of matched, new, jj tesla's for $69.00

Is this simply too little to spend on tubes? I've pretty much blown my hobby budget for the month.

Im a total tube NEWBIE so If I sound naive thats because I am!!

Recomendations on tubes. New vs used. expensive vs cheap. Upgrades and what NOT to use are greatly appreciated!

CHEERS :beerchug:
 
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Nice to know the 8B is working properly. The parts used on them just never quit. Amazing for a 50 year old unit!

As for the 7. Most likely you are dealing with a power supply issue. I am assuming the fuse blew. Correct?

Could be rectifiers, filter caps or both. I would re-stuff the filter cans to preserve the original look if they are found to be the culprit. The selenium rectifiers are hard to find NOS. A fella sells some reproductions on eBay. They work, but their long term reliability is unknown. Silicon would work, but at the risk of altering sonics or voltages if a series resistor is not used.

As for the power tubes on the 8B. RFT/Siemens are very nice without the hefty price of NOS Mullards. 1990's Winged C Svetlanas sound great on my 8B as well. Easy on the wallet, when you can find them!
 
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