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First post, first project RH84
First post, first project RH84
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Old 15th July 2009, 02:03 PM   #1
Dingleberryefxs is offline Dingleberryefxs  Finland
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Talking First post, first project RH84

First post, lots of questions. Thank you already!

My friend gave me a chassis from an old tube radio,
a big German-made
Blaupunkt florenz model:21351 from the early sixties.
After a closer look I noticed that it is actually a true stereo!
One EL84 amp section for each channel.

I've been wanting a long time to build a stereo tube amp and that radio would probably be a good platform to start getting into "hi-fi".
I've got experience only with tube guitar amps so this "hi-fi"-world is totally new to me, so please be patient.

I stumbled across the net and found that RH84-project and read lots of good about it. I also want to start with something simple because guitar amps and "hi-fi" amps are two totally different things. Less parts=less possibilities to go wrong.

It didn,t have rectifier tubes in it, but there are sockets for two 9-pin rectifier tubes so they have probably been EZ80 or EZ81.

I was wondering that is it possible to use EZ81 in RH84.
What I quickly calculated, one EZ81 would have enough juice to feed that amp, or am I missing something?

Power- and output ransformers look very sturdy, no rust or corrosion or anything, so they would probably do the trick nicely.
I'll have to measure power transformer voltages and output transformer impedances when I get to my workshop just to be sure everything really works as they should.

I've got source to get 22nf Jensen PIO caps for signal coupling. They will probably be very good choice, or am I right?
I can also get Solen Fast 47uf and 10uf caps for filtering.
Both at reasonable price.
How about that 220uf cap after the choke? Is there a certain brand you "Gurus" suggest? JJ 250uf/500V is easily available and
F&T 220uf/350V also. Or is there anything better?
Only 20H choke I can easily get from Finland is Hammond 20H/100mA. Quite big in physical size but 100mA is probably more than enough?

How about resistors? Metal film probably. Any good brand? I can get 0,6w 5% and 2W 1% metal films from the same parts source where choke and caps also so it would be easy to order all parts from the same supplier. Should I go with 2Watters?

I would also want to install a volume pot to it. What would be a good value? There is a 1meg resistor in the input. Is 1meg pot too much? 100K? Input impedance would certainly be lower with 100K pot, but will it have a major affect on sound? Alpha 100K and 200K stereo pots for "hi-fi applications" are available also from the same supplier. Any sonic differencies between tose two values?

Thank you already for patience to read this post.
All answers, information, advice, suggestions welcome.

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Old 16th July 2009, 08:14 PM   #2
Richard Ellis is offline Richard Ellis  Argentina
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Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
Welcome "Dingle'!
You seem to have thought out your amp well enough already...do you have a schematic for your 'discovery'?
Perhaps the fellow you got it from still has the old 'giant' wooden case?...sometimes the schematics are affixed on the insides ....a paper sheet glued to the inside???
We need that schematic so we can go forward with recommendations/observations et al.
I couldn't find anything right off the bat with a simple search...perhaps others deep into Blaupunkt gear will know of this amp.
__________________________________________________ Rick.....
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Old 16th July 2009, 11:03 PM   #3
Dingleberryefxs is offline Dingleberryefxs  Finland
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Hi Richard.

Worst thing is that my buddy didn't have neither the case nor the schematic. I have noticed also that it has been very common practice those days to glue the schematic inside the case...
I've done a lot of searching over the internet and hardly found any information about that model. Even the German radiomuseum site didn't have the schematic...
The RH84 schematic can be found here: http://www.tubeaudio.8m.com/RH84/rh84.html
But that's not actually a problem, because I'll be using only the transformers of that radio anyway. Of course I can try reverse engineer the amp section schematic out of it, it will just take time.
I've got sine wawe generator at my work so it won't be a big problem to calculate the impedance ratio of those output transformers so I can match the speaker load correctly.

And about the volume pot brand I meant Alps, not Alpha...
If I leave the 1M grid resistor in just to protect the tube if the pot happens to fail would the 250K be better? 1M and 250K parallel would make 200K total. 1M and 100K would make about 91K. Is the input impedance a big issue if you are driving it with low z source? Is 91K enough? As I told in my original post I've only been messing around with guitar amps and in that case the higher impedance would be better. Should I just stick the 250K pot and have no worries?

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Old 17th July 2009, 02:07 AM   #4
Ty_Bower is offline Ty_Bower  United States
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Here's what I've managed to gather about grid resistors. From the tube's perspective, the lower the ohms the better. You need to make sure there is no way you can build up a charge on the control grid. If it does, the condition will run away and the tube will die.

From the perspective of whatever is trying to drive the circuit, the higher the ohms the better. Your CD player doesn't like to drive a 10k ohm load. It's not built to source current.

If the pot is too big, the capacitive value of the interconnect combined with the load you are driving forms a filter which rolls off the high frequencies. A higher ratio of source impedance to load impedance reduces this effect and gives good frequency response.

The general rule of thumb is 1:10. So, if the data sheet says the 12AT7 requires at minimum 1 meg ohm grid leak, do not violate that. The RH84 schematic indicates 1 meg ohm in this position, so Alex runs it right at the datasheet limit. The 1:10 rule would suggest a 100k pot would be ideal for transferring signal from the pot to the input of the 12AT7.

Like everything else, there's plenty of room for wiggle. Your CD player will drive a 50k pot just fine. Likewise, a 250k pot will still give decent frequency response. You won't even notice the few extra kHz rolled off at the top end.
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Old 17th July 2009, 10:20 AM   #5
Shoog is offline Shoog
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Location: Eire
Just an observation. The RH84 is reportedly a good amp with good performance - but the driver is suboptimal as it has to low an output impedance and to low a current. Alex Kitic says he tried it with a pentode but had high frequency issues, but I suspect that what he did was use one of the low current small signal pentodes. These will solve the impedance issue but not the current.
I suggest looking at something like the 6AU6 as a driver. It will pass a healthy 6mA at high output impedance and will better match the circuit. I have used it in a very similar circuit and it works excellently.

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