5F6A First Power up test fail?

[GlenroseMedia]

First Build- “moderately modded” 5F6A, built in a Hot Rod Deluxe chassis, using HRD transformers. Added a 5v secondary transformer for 5AR4, wanting tube rectifier “sag”. Stock HRD transformer has no center tap for HV- so added a half bridge for ground reference. Then decided to full bridge before the 5AR4 for protection- ala robrob. Now the question:

Followed startup guide from Rob’s website, grounds etc. powered up with no tubes through variac and dim bulb (75w)- all voltages in spec. When I added the 5AR4 and did slow burn-in 10v steps through the variac, I got to 60v AC and the bulb started to glow, almost full brightness, then backed off some, but was limiting the voltage at about 55-60v AC, regardless of variac setting .
I removed variac from circuit, and about 20 seconds after power on, dim bulb did same thing; 5AR4 gets pretty hot, but no apparent glow. Also appeared not to be getting DC voltage correctly after the tube- even though the bridge diodes should have been providing.
Should an assume bad Rec tube? Is there a way to test? I can take the tube out of the power supply circuit, and just use the bridge to continue testing, but want to make sure there isn’t anything else less than obvious I should be seeing.
Sorry for long winded post- and thanks in advance for any advice!
JP

 

[sds1]

Hi JP, can you post a pic of your rectifier wiring? And yeah I think it would be useful at this point to know if the bridge is working by itself.

 

[2L man]

There is no need to use LBL if you use variac. Just measure that voltages increase when you gradually increase variac output few smaller steps to about 10VAC or so. Then you can go bigger steps and when you approach 50% mains tubes begin to be hot enough that they conduct and possibly this was what you saw when LBL began to glow? Bassman use so much current that LBL should have at least 100W bulp and still it will glow bright.

Have you measured that rectifier tube socket get AC voltage to correct sockets and other tubes sockets bit more AC to correct sockets? And that they were about proportional vs what you did feed to Mains input?

 

[GlenroseMedia]

Hi JP, can you post a pic of your rectifier wiring? And yeah I think it would be useful at this point to know if the bridge is working by itself.

Attached is concept drawing of power supply as is layer out. I will post pics an a few hours. Thanks

 

[GlenroseMedia]

There is no need to use LBL if you use variac. Just measure that voltages increase when you gradually increase variac output few smaller steps to about 10VAC or so. Then you can go bigger steps and when you approach 50% mains tubes begin to be hot enough that they conduct and possibly this was what you saw when LBL began to glow? Bassman use so much current that LBL should have at least 100W bulp and still it will glow bright.

Have you measured that rectifier tube socket get AC voltage to correct sockets and other tubes sockets bit more AC to correct sockets? And that they were about proportional vs what you did feed to Mains input?

Yes. Was considering a 200w bulb so as to get full current through. I did not check for proportional voltages; I guess I was concerned about powering down instead. Included in my drawing (but unlabeled) I do have a 1.5 amp fuse on secondary of PT, for PT protection, so I feel ok about powering up full with larger LBL…

 

[GlenroseMedia]

There is no need to use LBL if you use variac. Just measure that voltages increase when you gradually increase variac output few smaller steps to about 10VAC or so. Then you can go bigger steps and when you approach 50% mains tubes begin to be hot enough that they conduct and possibly this was what you saw when LBL began to glow? Bassman use so much current that LBL should have at least 100W bulp and still it will glow bright.

Have you measured that rectifier tube socket get AC voltage to correct sockets and other tubes sockets bit more AC to correct sockets? And that they were about proportional vs what you did feed to Mains input?

Oh, and to clarify- the rec tube was only one installed at this point

 

[andrewRneumann]

Just looking at your drawing quickly…. I think you need to disconnect the center tap of the rectifier filament supply from ground if you want this to work. Anyone else agree?

 

[Lowerleftcoast]

Just looking at your drawing quickly…. I think you need to disconnect the center tap of the rectifier filament supply from ground if you want this to work. Anyone else agree?

I agree, try it without the 5v CT

 

[sds1]

Just looking at your drawing quickly…. I think you need to disconnect the center tap of the rectifier filament supply from ground if you want this to work. Anyone else agree?

Agree.

Some quick Googling, seems like OP may have gotten a Hammond filament PT which come with the center tap.

 

[2L man]

Is there bias output on HV secondary but no Center Tab? That would be strange?

If rectifier filament coil has a CT its where HV is fed forward.

 

[2L man]

Yes. Was considering a 200w bulb so as to get full current through. I did not check for proportional voltages; I guess I was concerned about powering down instead. Included in my drawing (but unlabeled) I do have a 1.5 amp fuse on secondary of PT, for PT protection, so I feel ok about powering up full with larger LBL…

1,5A fuse is way too much for HV!!! 250mA slow blow is good for first test and if it won't burn when playing full volume install 300mA so there comes bit more reliability. This method can be continued one or two higher value fuses.

 

[sds1]

Is there bias output on HV secondary but no Center Tab? That would be strange?

Probably just a mistake in his drawing, the HRD PT has a separate winding from which the bias voltage is derived (along with a +/- rail).

 

[andrewRneumann]

Is there bias output on HV secondary but no Center Tab? That would be strange?

If rectifier filament coil has a CT its where HV is fed forward.

And that is probably a safer way to do it. OP beware that when you disconnect that CT it will have full HT voltage on it, not just 5vac.

 

[Lowerleftcoast]

Some rectifiers have the cathode connected to the heater element. This kind cannot have a heater CT connection.

5AR4

 

[GlenroseMedia]

Here are updates:
Yes-Hammond Filament Transformer, has CT. Disconnected, and proceeded with power up. Voltages are as follows:

No Rectifier Tube:
Line Voltage:119 vac
Bias voltage: 55.9 vac (yes- my drawing was incorrect- HRD has separate coil for bias, 1 side grounded?)
HV 337.7VAC
Rectifier heater 5.7 vac
Voltage to plates on rectifier: 207/195 vdc to ground (diode bridge)

With 5AR4 (actually GZ34) installed:
Line Voltage 118.5 vac
Bias 43.6 vac
HV 335.3 vac
Rectifier heater 5.39 vac
Pre/Pwr Heaters 7.01 (seems hi, but is in spec) (this winding is not CT; referencing through 100 ohm resistors)
B+, no load: 460 VDC

Will adjust HV Fuse and continue with adding power and preamp tubes.

 

[sds1]

Things are looking up!

 

[GlenroseMedia]

Sorry for not being clear- capping off the CT of 5v Transformer seems to have solved issue. Here are pics for reference.

Power Supply Section

Main Board (Hoffman, modified)

Whole overview:

Front panel:

I brushed the rough steel chassis and clear coated it. still unsure what to do with the three unused holes where the fx loop/footswith jacks were...

Thank you for the help and suggestions! (so far)

 

[dan40]

Congratulations on a successful build!

still unsure what to do with the three unused holes where the fx loop/footswith jacks were...

I normally use chrome hole plugs to fill empty spots. You can find them at most hardware stores and online.

https://www.lowes.com/pd/Hillman-0-375-in-Chrome-Plated-Steel-Hole-Plug/3013212?cm_mmc=shp-_-c-_-prd-_-hdw-_-ggl-_-LIA_HDW_126_Fastening-_-3013212-_-local-_-0-_-0&ds_rl=1286981&gclid=CjwKCAiAh9qdBhAOEiwAvxIokwP23cekvjMXvRfmgruDB-NSgh5WFSz-rggFWIz1ukheoZWwrb0qjhoCGJ0QAvD_BwE&gclsrc=aw.ds

 

[BigDaddy23]

Nice build! What are the switches doing? First one looks like switchable V1 bias, second one bright cap? Can't quite make out the last 2.

 

[GlenroseMedia]

Nice build! What are the switches doing? First one looks like switchable V1 bias, second one bright cap? Can't quite make out the last 2.

v2 bias, bright, raw tone, and negative feedback (hot/cold/none)- all from RobRob