Bassman 5F6-A voltage check

[Shamadan]

Hey guys , I have the 5f6a clone (weber layout) up and running Sound is Great but I have few doubts that You may solve.
I used transformer shunt method to check bias current 16.8 mA was the value I got.
Tubes GZ34, 6L6GC, ECC83S JJ
Voltages measured without power tubes
V1, V2 6L6GC JJ
Pin3 450
Pin4 450
Pin5 -50
Pin6 450

V3 ECC83S JJ
Pin1 254
Pin2 28
Pin3 48
Pin6 246
V4 ECC83S JJ
Pin1 186
Pin3 1.27
Pin6 337
Pin8 187
V5 ECC83S JJ
Pin1 225
Pin3 1.85
Pin6 221
Pin8 190

May Questions
Do I need to change bias current if yes to what value. (I used bias 56k resistor? as on weber layout)
What are the benefits of increasing the current
I will use amp with bass and harp (I checked it with acoustic guitar and sound was great)
Are the voltages acceptable.
Btw. The Tubes glove is very weak can I somehow make them more bright.
I am using Hammond 290DEX Power Transformers
Thanks in advance for your help

[Enzo]

Glove? Did you mean glow? If the amp makes sound, then the glowing heaters are working. They don;t need to be brighter.


Everyone has his own idea what bias ought to be, but may I say this:
You say it sounds great. If that is so, then leave it alone, it works and sounds good. NO point in adjusting it or changing it if you like the sound.

[Shamadan]

Thanks for reply
I can see them glow but i wasn't sure about the brightness.
Is bias current responsible for how loud the amp would be. Sorry for such a trivial question
Sound is amazing but want to make sure it would stay like this for a long time,
so am trying to make everything right.

[g1]

Bias will not have a great effect on volume. At extreme settings (hot or cold) bias will have an effect on tone and reliability of the power tubes. 17mA bias sounds quite cold but you did not say what the plate voltage is with the power tubes installed.
For example, with a plate voltage (pin 3 for 6L6) of 425V and current of 17ma you would have about 7 watts idle dissipation (425 x .017).
A modern 6L6 tube is rated for 30W, so 7W would be about 23% idle dissipation. You will probably want to be at least 50% to avoid crossover distortion. Ballpark bias ranges are usually anywhere from 50 to 70%. It is usually a compromise between reliability and tone. Lower % give longer life for the power tubes, higher % may give better tone.
You need to measure the idle current and the plate voltage for the calculation as the voltage will change depending on the bias setting.

[Enzo]

The stock Peavey 5150 head will typically run between 11ma and maybe 20ma. SO that is mostly lower than your 17ma, and the B+ is higher, so much cooler than yours. I still say, if it sounds good, stop fixing it.

[loudthud]

Is that a typo on pin 8 of V5? 1.9V seems more likely.

[Shamadan]

To many typo. Yes 1.9v. I somehow mange to build this amp without major mistakes. Now will try to understand why and how it works.

Thanks for everything.

[Shamadan]

Hi
A small Problem has occurred yesterday with my 5f6a build. While playing bass and acoustic guitar for 1h on low volume some distorted sound appeared 3 times for 10 seconds each. It was strange because this distorted sound sounded like listening to rock music on bad laptop speakers. Kinda hard to explain.

I decided to change bias to 35.5 mA, V1,V2 pin3 414V pin5 -39V power tubes in. (does the numbers look fine?) Do you think that I may have heard crossover distortion or should I look for something else.
Btw. My bass has some problem with grounding/shielding because when I touch the 5f6a chassis or strings some noise disappear.
Thanks so much for your help.

[Tom Phillips]

...While playing bass and acoustic guitar for 1h on low volume some distorted sound appeared 3 times for 10 seconds each. ...Do you think that I may have heard crossover distortion or should I look for something else...

Your description of three 10 second episodes of distortion over a one hour period is more consistent with an intermittent connection or part failure. It could be caused by lots of things such as dirty or loose contact, flaky solder joint, bad tube etc. A specific bias setting will not cause this type of symptom. When it happens again try thumping on the cabinet with your fist to determine if it makes it stop or get worse. Also rule out any other stomp boxes you may be using by plugging directly into the amp. It's also good to verify that it's not your guitar cord. Just try another one. Sounds like you already ruled out the guitar because you said it happens with both the bass and the acoustic.

[g1]

My bass has some problem with grounding/shielding because when I touch the 5f6a chassis or strings some noise disappear.

It is normal for the hum/noise to get quieter when you touch the strings or chassis.

[Shamadan]

Hi
This distortion first occurred with only C# note (acoustic) it sounded like "farting" speaker. All other notes were ok.
Then I was jamming with my brother acoustic and bas together plugged to 5f6a and after 10 minutes or so speaker start farting again for those 10-15 seconds on different chords. I was suspecting the speaker but it is kinda difficult to find the error because now distortion last for few seconds per hour - to little time for me to locate the problem. Any Ideas?

[Shamadan]

Hi
Sorry to bother you again, but I have one last question that may solve my previous problems. When I plug an instrument (bas or harp mic) to bright inputs they are not 100% quiet when my volume is set to 0 (all the way down) normal inputs and pot works fine. Is it normal for bright inputs to act like that. When I use green bullet mic and set vol pot on mic to max and bright canal to 0 the amp is quite loud. I check the wires but they seems fine. Any ideas?