Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on May 24, 2017, 06:59:24 pm
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PT has HV CT, no R tube, full wave diodes, fixed bias, no bias tap from PT.
How do I get a fixed bias tap, I've done this with a bridge but not with a full wave.
Schematic would be helpful, diagram, etc.
Thanks,
al
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Copy the Princeton Reverb.
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Like Sluckey said.
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Thanks,
al
Edit: I noticed my old bridge rectifier no CT, had a .047 Mallory coupling cap between the HV wired tapped and the diode? I did this a while a go and can't remember why?
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Edit: I noticed my old bridge rectifier no CT, had a .047 Mallory coupling cap between the HV wired tapped and the diode? I did this a while a go and can't remember why?
That's just the way it is. :icon_biggrin:
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Great answer :laugh:
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Another one.
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And one more.
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I have noticed that people view the .gif file but do not download the .pdf file. The .pdf file contains a table that goes along with the schematic.
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More.
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Thanks 2deaf,
Any way to salvage a filter cap with a broken lead, axial type. I have a new never used, very small 22uf/450v cap with the neg/common end broke off. Very clean break.
I once tried to solder a common lead to another broken lead, worked for a while ... seems to not be reliable.
I guess it's trash...?
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IN an emergency you can create a blob of solder and keep the end in that and it 'should' work, put extra flux on it, but consider it dead and buy a new one. If you HAVE to it may work for a week or two, but it will likely vibrate away after some short time. In other words, if you need it for a gig today it 'may' work short term, but otherwise consider it garbage.
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The case is Aluminum. Reliable soldering to aluminum requires special flux and technique. It is rarely done.
The original lead was spot-welded, probably before they put the guts in?
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Thanks, it's trash.
Just that I never saw such a small 22/450 cap, 1" by 1/2".
I needed it for a small spot, forgot where I got it.
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Just don't fuse the CT line when using such a bias tapping circuit - fuse each HT leg after the tap.
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If I use this bias tap, lower schematic with two 50uf caps, 15K between, what ohms is the load resistor?
600vac PT, 300-0-300 w/CT
I had a 50K variable bias pot, 33K to ground, gave me correct bias range when I had a bridge R. Around 24mA for two 6V6s was the middle range.
What does a load R do?
al
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> What does a load R do?
Breadboard it (tack it together) and try it.
This plan needs some loading. The exact amount is very tedious to calculate. Just do it.
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I had a 50K variable bias pot, 33K to ground, gave me correct bias range when I had a bridge R. Around 24mA for two 6V6s was the middle range.
I'm not sure how you have it hooked-up. I don't know what you mean by a "bridge R".
What does a load R do?
Like PRR said, you need a particular load to get a particular bias voltage. It is not immediately obvious how to calculate what voltage will result from what load, which kind of leaves you with trial and error. My tables allow you to start with a load that is at least in the right ballpark. The tables are also useful for designing a range with a variable bias.
For example, let's say you want -37V for your bias with a 300Vrms PT. The table shows -33.9V with a 27K load and -40.5V with a 33K load, so a 30K load should result in about the desired -37V. For a variable supply, we would set the center point of the pot. in series with a fixed resistor so that the total load is equal to 30K. So if we had a 10K pot. in series with a 25K resistor as shown below, the total load at center point would be 30K. With the wiper at one extreme, the total load is 25K and at the other extreme, 35K. Looking at the table with my thumb at arm's length, it looks like 25K total load would yield about -32V bias and 35K would yield about -42V.
If we used a 25K pot., an 18K resistor would give a total load close to 30K with the pot. at center point. The extremes are 18K and 43K with their corresponding bias voltages of about -23V and -50V.
The advantage of an arrangement as shown below with a pot. in series with a resistor is that the bias defaults to the coldest setting if the wiper fails open.
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What a dumbo, me that is, ha.
Your drawing are fixed bias, meaning set with a resistor not a pot. The load resistor in this case sets the negative bias, not sure what I was thinking.
Actually very simple, thanks for the explanation. I guess my lack of any mechanical engineering schooling, limits my understanding of terms and principles. After a while stuff finally sinks in and I feel like a dumbo.
"Bridge R", meant to be a bridge rectifier. The original PT in this Crate conversion (I'm changing to Sluckeys Deluxe Lite now) had no HV CT and I changed the Crate to fixed bias from cathode. So , I used four diodes making a bridge rectifier. I tapped one leg of the bridge to get the bias feed, I had a 56K dropping R, diode, filter then to a bias pot as a variable resistor to get the required voltage. Since the new PT I'm using has a HV CT, I used a full wave rectifier and wasn't sure how to get the bias tap from the full wave but now I see it's basically the same as I did for the bridge. My question about the load resistor size was kind of a dumb question, like PRR said, "just do a breadboard" as now I see there is no answer to my question, all depends on what bias voltage you need. Now I see the simplicity....
Thanks,
al