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 Post subject: Bogen DO70
PostPosted: Jan Mon 01, 2018 11:16 pm 
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Am starting a restoration of one and am wondering what anybody out there might know about them.

I have the SAMs but it seems to be an earlier version. Any info appreciated.


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 Post subject: Re: Bogen DO70
PostPosted: Jan Tue 02, 2018 10:49 pm 
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Anybody?


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 Post subject: Re: Bogen DO70
PostPosted: Jan Tue 02, 2018 11:59 pm 
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What are you going to do with it, convert to guitar or hifi? Was that a PA amp to start with?

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 Post subject: Re: Bogen DO70
PostPosted: Jan Wed 03, 2018 3:57 pm 
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Seems we need a thread on the DO70.

The DO70 is a mono hifi amp from about 1956.

https://www.radiomuseum.org/r/bogen_do70d07.html

They are somewhat rare, in that I have seen very few of them. I looked for 7 years to find one to match my Bogen R775 tuner/preamp.

http://www.classicaudio.ru/images/oldpages/image/bogen_r775.jpg

At the time the DO70 was their top of the line amplifier. It puts out 70 watts. One neat feature of the case is that the legs can be mounted on the bottom or the back, so it could be layed down or stood up.

I intend to fully restore it to original operating condition. I might use some nice coupling caps, but I don't like to do 'modifications' to my vintage audio gear. I think they are pretty useful just the way they are - I mean were. :)


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 Post subject: Re: Bogen DO70
PostPosted: Jan Thu 04, 2018 3:59 pm 
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Started the restoration last night.

As I said I have the SAM's, but going through the circuit this earlier version has the same iron, output tubes and rectifier, but is otherwise a completely different amp. Ah well.

I'm starting with a complete recap. The only cap I don't have on hand is the AC input cap to ground. I will order a safety cap and replace it when it
comes in.

The output stage coupling caps are 600V in this unit, 400V in the schematic. Looking at the circuit, the driver tube has 285V on the plate and output grid quiescent control voltage is -35V, which adds to just over 300V. I can use the 400V caps I have on hand to check overall operation. I will order some good audio 600V coupling caps and put them in soon.

After looking at the PS wiring situation, I have restuffed the multicap can, something that I rarely do. It was just going to be too awkward to wire in the Axial caps, much easier to restuff the can and wire back as original.

There have been a few resistors that are slightly over the tolerance limit. I am putting together an order and will replace those when they come in.

Just one more tricky job, and that's replacing the Selenium rectifier. It's not only in a somewhat awkward spot (screwed at one end in a rather crowded space) but I also have to get the dropping resistor right. The schematic shows the voltage is dropped through a 100K resistor to each output grid, but in the unit I have the bias goes from the smoothing cap straight to the grids with NO isolation or dropping resistors at all. How does that even work? I must have traced it wrong.

So we'll see how it goes. I think I replaced a similar SR in a Bogen RB140 and added a 2K resistor. I'll start there and see how it goes.


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 Post subject: Re: Bogen DO70
PostPosted: Jan Thu 04, 2018 5:54 pm 
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Location: Rochester NY USA
Usually the bias voltage is made adjustable, so must be higher than required - no voltage diver required here. The voltage drop through the selenium will be small, since there's essentially no current. Voltage won't be much different with the silicon replacement - might be a hair more negative, but screen voltage is adjustable to set idle current. I'd change the 33 Ohm cathode resistors to 1%. (I'd use 10 Ohm since I have 'em, and cathode current is easy to calculate: 30 mA is 0.3V etc .) The 15 mA shown seems awfully low for idle, only 10W dissipation. (Maybe a trifle higher, once the cathode resistor's 0.5V drop is jumpered out). Obviously the power transformer is good for more than 46 mA. (Sams just measured the idle current.)


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 Post subject: Re: Bogen DO70
PostPosted: Jan Thu 04, 2018 9:29 pm 
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Tom Bavis wrote:
Usually the bias voltage is made adjustable, so must be higher than required - no voltage diver required here. The voltage drop through the selenium will be small, since there's essentially no current. Voltage won't be much different with the silicon replacement - might be a hair more negative, but screen voltage is adjustable to set idle current. I'd change the 33 Ohm cathode resistors to 1%. (I'd use 10 Ohm since I have 'em, and cathode current is easy to calculate: 30 mA is 0.3V etc .) The 15 mA shown seems awfully low for idle, only 10W dissipation. (Maybe a trifle higher, once the cathode resistor's 0.5V drop is jumpered out). Obviously the power transformer is good for more than 46 mA. (Sams just measured the idle current.)

Thank you for your comments Tom. I take it your measurements are from a DO70? Unfortunately there are no adjustable voltages in this amplifier - I seem to have the lesser version.

At any rate...

Initial testing has gone well. There are only four paper caps in the entire amplifier, so I decided to leave them be and buy specific caps for each place.

I had definitely lost the bias wire in the bundle late last night. It actually goes to a tie point where the two 200K bias resistors split as expected. They measured within spec but at different points. I will be ordering a new set.

After recapping the power supply and replacing the Selenium Rectifier (2K ohm dropping resistor) I was fortunate to find a new two-wire power cord roughly the same dimensions as the original, which was badly deteriorated and had been cut off. After cleaning all contacts, switches and pots, and taking care of a few other details, I decided everything looked good so it was time for the smoke test.

I run a big mono system in my shop, so it was no problem disconnecting the regular power amp and connecting the DO70. The music selection for the initial test was Martin Denny's 'Exotica', which I figured was the correct era for this 60 year old amp, and would no doubt have had a place in every swinging bachelors collection at the time. :mrgreen:

One oddity of the amp is that there is no power switch. It is obviously intended to be run from a preamp with switched power. No problem, just... odd.

It performed very well. Because the amp was set on the speaker, and I initially built the speaker for testing purposes and bracing was not a high priority, when I turned up the volume there was a noticeable intermittent buzzing, due no doubt to tube microphonics. Not much I can do about that, but will continue to investigate.

The Mullard reissue EL34s (matched pair) I bought for it had plenty of power and detail. Overall I'm very happy, and listening to it now. It does put out a lot of heat, but seems stable. If I can figure out another hosting site I will post some pictures. Can't wait for step two of the restoration! :D


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 Post subject: Re: Bogen DO70
PostPosted: Jan Fri 05, 2018 5:19 pm 
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Location: Rochester NY USA
I was looking at the Sams folder - it shows an adjustable screen voltage regulator. Perhaps that was eliminated?


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 Post subject: Re: Bogen DO70
PostPosted: Jan Fri 05, 2018 7:08 pm 
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Tom Bavis wrote:
I was looking at the Sams folder - it shows an adjustable screen voltage regulator. Perhaps that was eliminated?

Yes, it was.

Actually, I'm not sure which came first. I know the DO70 was one of the last of that era amplifiers sold, as it is shown in the next generation catalog, while nothing else of that style is. So, I guess I'm going to have to say I have one of the later ones.

My understanding is that when something is published in SAMs it is a manufacturing release, so it will be published when it is first manufactured. Bogen seems to have had a policy of going back and redesigning things practically the next production run. Not sure why that was, but it is very frustrating when you have all these SAMs but they only apply to the first production run, not the ensuing 2 or 3 years they were produced and sold.

I'm thinking I'm going to have to procure an IM tester to make sure it is working properly. Anyone have a Heathkit AA-1 they'd like to get rid of?

Last night after listening to Martin Denny I decided on a little more testing. I next selected 'Return to Paradise' by 'The 50 Guitars of Tommy Garrett', another period correct album - which, coincidentally, boasts another wonderful rendition of 'Quiet Village' - but I like this record because something in the way they recorded it just sounds so clear and super-present. It's like Phil Specter's 'Wall Of Sound' put to pop orchestral music.

That sounded so good I decided one more test was in order. This time I decided to let the amp stretch it's legs, so I put the amp on a shelf next to the speaker, and this time selected something modern. Here is a sample: https://www.youtube.com/watch?v=HHdylh210Ho

I cranked it, with a good helping of bass boost. It really gave that JBL 2226H a workout. I could really feel the bass. I listened to the whole album, and i have to say that amp got HOT. The power transformer was hot, the output transformer was hot, the case was hot, the chassis was hot. It really did sound great though!

After that, I decided perhaps the liberties I was taking with a half restored 60 year old amplifier were a tad on the risky side, and restored connection to my regular amplifier, an Adcom GFA 6002 run in bridged mode.


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