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Old 9th June 2013, 02:51 PM   #1
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Default Bucking and boosting a tube power transformer

I'm posting this thread to help out another member (Ranhaber) building a Tubelab SE.

He is in 220V land and has a James 9612 power transformer with 230V primaries, and his secondary voltages are measuring low even with the transformer unloaded.

The 9612 has lots of 5V and 6.3 volt taps, and I've suggested that he can boost the secondary voltages by using one of the spare 6.3V taps in series with the primary winding, or perhaps both a spare 5V and 6.3V winding for even more volts.

Is this the best approach and is it safe? I realize that some isolation is sacrificed doing this; is there a better way?

All of the gory details are in this thread starting at post #58:

Tubelab SE
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Old 9th June 2013, 04:09 PM   #2
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Remember, boosting the primary increases the voltages observed at all secondaries used "normally".

If this is an accurate description of the James' secondary situation, configure S4 and S5 for 5 V, phase them up, and wire them in series. Use that composite to boost the primary.

Regulation effects will probably lead to a boost slightly greater than 10 V., but that's the best that can be done, without acquiring additional "iron".
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File Type: jpg Buck_Boost Arrangement.JPG (233.7 KB, 207 views)
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Old 9th June 2013, 05:00 PM   #3
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Originally Posted by Eli Duttman View Post
Remember, boosting the primary increases the voltages observed at all secondaries used "normally".
Thanks Eli! He needs to raise all of the secondary voltages.

So, what you are saying is use two 5V windings in series and put these in series with the primary........If the phasing is correct we get a 10V boost multiplied to the secondary. If one of the 5V is flipped we will get zero boost I would assume, since they would cancel.

No worries about isolation?
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Old 9th June 2013, 05:24 PM   #4
FoMoCo is offline FoMoCo  United States
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Remember that by doing this you're pushing the transformer closer to saturation. If it was marginally close to start with you'll see core losses go through the roof. I'd power it up unloaded and see if it gets warm before calling it a done deal.
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Old 9th June 2013, 11:17 PM   #5
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There is highly likely to be an isolation problem with using a secondary winding on the primary due to the safety related insulation and creepage/clearance design of the transformer. The safety issue could only be clarified by a design inspection, which if you weren't in contact with the designer could involve a tear-down. Some transformers even have a safety earth screen that physically separates all primaries from all secondaries, whereas others use separate winding sections with double-insulated safety rating that don't per se need a protective earth.
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Old 10th June 2013, 02:40 AM   #6
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Quote:
There is highly likely to be an isolation problem with using a secondary winding on the primary due to the safety related insulation and creepage/clearance design of the transformer.
I can't argue with those remarks. The 100% safe way to boost that James primary is with a separate 10 VAC "chunk of iron". A Hammond 266L20 will get the job done.
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Old 10th June 2013, 07:01 AM   #7
12E1 is offline 12E1  United Kingdom
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Originally Posted by Eli Duttman View Post
I can't argue with those remarks. The 100% safe way to boost that James primary is with a separate 10 VAC "chunk of iron". A Hammond 266L20 will get the job done.
Agreed. But to avoid the possible saturation issues referred to (when using the boost configuration), put the buck or boost winding from the additional transformer in series with the secondary of the main HT power transformer. That way the HT transformer is known to be running within its design parameters.
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Old 10th June 2013, 07:48 AM   #8
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Originally Posted by 12E1 View Post
Agreed. But to avoid the possible saturation issues referred to (when using the boost configuration), put the buck or boost winding from the additional transformer in series with the secondary of the main HT power transformer. That way the HT transformer is known to be running within its design parameters.

Boosting the rectifier winding is fine, when bridge rectifiers and "full wave" doublers are employed. Unfortunately, the James power trafo in question is configured for FWCT rectification. Boosting the primary is the simple option. Also, the filament windings are reading low. Once again, boosting the primary is the way to go.

To avoid saturation, the builder will have derate the VA number of that James "chunk of iron". Not using S4 and S5 should get the job done.
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Old 10th June 2013, 07:57 AM   #9
AJT is offline AJT  Philippines
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Quote:
Originally Posted by boywonder View Post
I'm posting this thread to help out another member (Ranhaber) building a Tubelab SE.

He is in 220V land and has a James 9612 power transformer with 230V primaries, and his secondary voltages are measuring low even with the transformer unloaded.

The 9612 has lots of 5V and 6.3 volt taps, and I've suggested that he can boost the secondary voltages by using one of the spare 6.3V taps in series with the primary winding, or perhaps both a spare 5V and 6.3V winding for even more volts.

Is this the best approach and is it safe? I realize that some isolation is sacrificed doing this; is there a better way?

All of the gory details are in this thread starting at post #58:

Tubelab SE
i am really scared even to try bucking or boosting at the primary side, i have done it at the secondary side, i will do it anytime it needed to be done...
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Old 10th June 2013, 08:07 AM   #10
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Hi,

I tried to boost the primery like in the picture below.

For a second i mesured the same 4.5V and 6V (expected 5V and 6.3V) as in normal connection, and then 0V for all.
The taps i mesured were the bottom ones (4A each).

I pulled down the voltage and that's it. (my girlfriend insisting that i go to bed)

p.s. the fuse looked ok.
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File Type: jpg primary(220V).jpg (90.1 KB, 139 views)
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