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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Tue 03, 2018 4:19 am 
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A simple attenuator can be made by putting a resistor in series with the inputs to the amp.

The resistor value will depend on the input impedance of the amp and the desired drop in signal level.

If for instance you have a 1 volt signal and wanted a 500mV signal you would make the series resistor equal to the input impedance of the amp.


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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Wed 04, 2018 3:31 pm 
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Location: Kalispell, Montana
Hi Steve, it has been a while.

This thread is fascinating as I have and do encounter this same exact problem. I have an amp that needed the edge, so to speak, taken off just as you described.

The preamp tube was a 12AX7, which has an amplification factor of 100. So what I did was replace it with a 5751. It is a modified 12AX7 and has an amplification factor of 70. It does draw 25ma more heater current but the power transformer can handle it just fine. I have to tell you, it did make a difference and took that "edge" off.

I am not certain in the long haul if I will keep it that way or do something at the inputs like you did. Perhaps I will compare both and see what works best. I just wanted to reply and say, that in my case, a tube substitution did work.

Best regards, Steve

Tony


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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Wed 04, 2018 5:05 pm 
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Think the 5751 is a little quieter or less microphonic (forget which) than the 12AX7.


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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Wed 04, 2018 8:49 pm 
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Hi TR,

The 5751 has less gain (Mu of 70 vs. 100 for the 12AX7) so it could be thought of as being a bit "softer". It is called a special version and it is more rugged as well so it could be less prone to microphonics but I really do not know about that. I was using that example from an experience of mine to give Steve an idea.

Steve, the 6AU6 has a Mu of 38. The other tubes you mentioned (the ones I just picked and looked up in my GE manual), though can be substituted, have amplification factors that are greater than that but perhaps further research is necessary as I did not look at all of them. You might also take a look at the 5749 and 6660 as they are special versions of the 6AU6, but I do not have any data on their Mu in front of me at the moment. Just a thought.

I'll be curious on what you decide upon. It is giving me some ideas, too.

Tony


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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Thu 05, 2018 1:40 am 
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hi tony,

my options are either to make an Lpad with a 1meg log pot, which i have, to match the volume control.

my second option is to roll a tube which would be the easiest (i really like my easy chair on these cold april snowy evenings) :) .

i won't use a resistor, either in parallel or in series, because when i did this to 'match' some tetrad cartridges to a tube amplifier, it really messed with the tonality, especially the low end.

otoh, the 50k volume control that i purchased from amazon worked ok, but i believe it too altered the tonality a bit as well, now that i removed it. using a 50k pot instead of the (possibly upcoming to be made) 1m pot is quite a difference.

so, those are the two options i have, where rolling a tube will be the easiest option.

if you can come up with the gain of the mentioned tubes, i would appreciate it. since i have cleaned out the shop, downsized 98 percent and now own about 35 tube pieces and about 20 vintage solid state pieces, all my books are put away into storage.

in the meantime though, i will look up the data on the internet, which i would trust a book's numbers better than the internet...lol.

steve

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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Thu 05, 2018 11:22 am 
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The issue with using a resistor in series with a caermic cartridge is that it changed the cartridge's loading which of course would have affected tone.

Using a resistor on a regular line input should not affect tone.


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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Thu 05, 2018 8:26 pm 
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Hi Steve,

I agree with what TR said above.

I had some time so I went through my GE and RCA tube specification manuals with every drop in substitute that could be done without modifications. Unfortuately in this case, the 6AU6 has the least Mu of any of them, so if you wanted to drop a different tube in it would probably make no difference other than to perhaps increase the gain.

So this does limit options to the device you are describing making. I have used resistors for line inputs with success and no loss of tone. I have also tried them with ceramic cartridge inputs and was not very pleased for the reasons that TR mentioned.

This may sound nuts but bear with me here. If you have a few extra 6AU6 tubes or equivelent substitutions, test and pick the weakest one. It may work well with weak emission and it will have less gain. Another solution I did one time with a Magnavox amp connected to a record player that sounded overdriven was I put in two 10 watt 15 ohm resistors, one on the hot side and one on the neutral, to drop the line voltage. It dropped from 123 VAC to 107 VAC. They run around 170 F so need to be ventilated, but it dropped the B+ as well as filament voltages...actually slightly below where they should have been as the amp was designed for 117 VAC. The tubes and transformer run cooler and no change in output, though it sounds so much better.

Just a few ideas there, Steve. I know they sound odd, but it has worked.

Let me know what you do. I am interested.

Tony


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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Thu 05, 2018 8:28 pm 
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One thing that may work is to insert a resistor in the heater circuit of the 6AU6 tube which will drop heater voltage some and drop the gain.


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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Thu 05, 2018 9:24 pm 
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Location: pensacola fl
Ah the old starved heater trick it works but a better way is if the cathode is using a resistor to ground with a bypass cap usually a small electrolytic disconnect one end of the cap and insert a pot that is several time higher than the cathode resistor and adjust the gain as needed. This way the dc operating point is not changed. After you find the gain you need take the pot out and measure it and put in fixed resistor.

Bob


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 Post subject: Re: Match Line Level to Amp-Use an Attenuator or Roll a Tube
PostPosted: Apr Fri 06, 2018 1:32 am 
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thanks for all the input, guys. i will do either of the following:

1. try various values of a series resistor to get the 'match' that i want, since it all say that it won't alter the tonality as i experienced once with a tetrad cartridge and another tube amp--that was just horrible. it was either distorted and bassy or normal and cellphone tinny sounding.

2. make the 1 meg pot so i can adjust it, as in "set it and forget it".

this unit is all put together as seen in the thread link above.

taking it apart is not an option at this point--i just don't feel like doing that.

i only have a nice pile of NOS 6AU6 tubes, so there are no weak ones.

i will advise, but not very soon, perhaps a few weeks or so.

thanks,

steve

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