5F1 - hum and speaker crackle

[electrochronic]

There are other problems with these 5F1 amps if you are not running current production JJ tubes for the Power output tube and rectifier tube. Most of the PT transformers sold today in USA are spec'd for 117Vac mains voltage. What is the problem with this you might ask , well mains voltage in California is 120vac. The trasnformer is being fed 3vac more volts on the primary than what the transformer was designed for. It will still work , but the issue is that the secondaries are elevated ("x"vac = "x" vac increase on secondaries). This is not a problem so much on the HT side if your running current production JJ OT 6V6 as much as it is a problem with 6.3vac heaters voltage which generally sees as much as .5 volts increase to your tube heaters which is not good at all. Moreover , the 5vac secondary for the rectifier heater sees .5-.7 vac increase as well. This all means the heaters are receiving too much voltage wether your running new production tubes or NOS tubes. This really becomes a issue when you want to run a expensive set of NOS american or european made vintage tubes to get your vintage tone. These overvoltages are stressing those heaters on these expensive tubes over their design ratings. There are two solutions that will remedy this.

1. Use a bucking transformer (ex: 120vac to 6.3vac) in conjuntion with your PT that will lower mains voltage so the secondaries fall in the correct range. Use of a auto trasnformer to determine the correct bucking transformer to use is suggested. The bucking transformer must be rated for the same current as the mains fuse, but not lower.

2. Use a custom made transformer that is specified for the mains voltage that
the amp will be in use. The problem here is if the amp will travel from town to
town then mains voltage may vary. In this situation, a PT with several mains taps, the mains taps on the PT will need to be switched for three settings 115, 117, 120 to meet internal voltages for maximum tube plate voltages etc. You'll also have to carry around a voltmeter to check the AC mains before setting up the amp for the proper line tap. this is a hassle.

If your in a place with 220-240 mains voltage the same rules apply but use different mains or a step down transformer if your amp runs on 1XXvac voltage.

Lastley, this is trouble with unregulated circuits.

If some anyone says its normal for a NOS 6v6 tube to run on a 410+ plate voltage then go look at the maximum published specs and you will know the answer. If you bench test this you can confirm it , too.
In this case a NOS 6V6 is ~350vac max. for NOS tubes. The JJ 6v6 is probably like ~450+vac , no issue with this one. However this still does not eliminate the heater overvoltages to the 6v6, 12ax7, and 5Y3 which are just as important for the tubes NOS or not.

Also , I have noticed that little tricks like dropping resistors in the circuit to get the voltages down only lower whats left of the reserve current on the PT because resistors are current dropping devices too. this is not a efficient way to get these overvoltages corrected. However if using say a 1 ohm reistor to get a heater voltage down , this should not be a problem. I 'm really addressing resistors in the 100 ohm or above range here. These are the ones that will drop significant current and will only lower voltage slightly. The main point here is too keep reserve current as high as possible to correct for sagging. If the reserve current is low and there is not sufficient reserve current the power supply will ripple and suffer, the transformer will probably run hot, and the audio quality from the amp will suffer.

Back Biasing with a zener diode only works well for reducing the HT secondary vac, but does not correct the heater overvoltages.

[Bruce / Mission Amps]

...Most of the PT transformers sold today in USA are spec'd for 117Vac mains voltage. What is the problem with this you might ask , well mains voltage in California is 120vac. ...

My bench voltage is 124vac right this min... and it goes anywhere from a low of 122vac to 126vac when the loads are down.
That's a time killer when trying to set idle current and the guy goes home and his line voltage is 115vac-117vac in some old rat trap house.
I use my variac now and set it first to 122vac before any power tube idle current setting.

[electrochronic]

My bench voltage is 124vac right this min... and it goes anywhere from a low of 122vac to 126vac when the loads are down.
That's a time killer when trying to set idle current and the guy goes home and his line voltage is 115vac-117vac in some old rat trap house.
I use my variac now and set it first to 122vac before any power tube idle current setting.

Bruce

This does'nt surprise me on bit, in fact I went across county today to pickup some caps for a project to find out its 118vac there. So, I think I would rather have my amp running on the low voltage side than the elevated voltage side. It seems each amp has to be built to a mains voltage you can settle with in your area. I noticed that the mains here do not drift upwards
of 126vac , thats pretty high.

[Bruce / Mission Amps]

Bruce

This does'nt surprise me on bit, in fact I went across county today to pickup some caps for a project to find out its 118vac there. So, I think I would rather have my amp running on the low voltage side than the elevated voltage side. It seems each amp has to be built to a mains voltage you can settle with in your area. I noticed that the mains here do not drift upwards
of 126vac , thats pretty high.

I called the power company and they say their specs are +- 5.8%... with a max of 127-128vac. Over that and they claim they will come out and replace the distribution transformer.
Yeah right.... just think of all the free money they get from extra current used up at 126vac vs 117vac.

[MWJB]

"If some anyone says its normal for a NOS 6v6 tube to run on a 410+ plate voltage then go look at the maximum published specs and you will know the answer." What like 55mA @ 400v (corrected) for class A operation, according to the RCA data sheet? You need to re look at the specs.

"Most of the PT transformers sold today in USA are spec'd for 117Vac mains voltage. What is the problem with this you might ask , well mains voltage in California is 120vac." Hmmm, you mean they're spec'd just like the original transformers that they replace? That's what they are supposed to do!

"This all means the heaters are receiving too much voltage wether your running new production tubes or NOS tubes." Piffle. The data sheets, that you claim that we should be observing, state 6.3VAC-6.9VAC.

"In this case a NOS 6V6 is ~350vac max." You should tell Leo Fender, he obviously built all the tolex 6V6 amps wrong! While you're at it, call Mark Baier and tell him how to build 5F1s...with the 5V4 rectifier option they show 404v uncorrected on the plate...sound damn good too!

No harm in using a bucking transformer, or getting a custom PT wound, you're welcome to do (it's your amp do what you like with it) that but knock it off with the scare mongering. None of this is "news", or a "revelation", or indeed relevant to the vast majority of 6V6 powered amps.

Tube data sheets are writen by tube manufacturers, amps are built by amp manufacturers...they're not working to the same agenda.

[electrochronic]

No scare mongering, here. Just trying to get into the details of this amp that most builders do not pay a whole lot of attention to , its the details regarding the HT voltages mostly.

just stating facts which I have read and bench tested, which no one really seems to regard as noteworthy. It seems to me that a good design should meet two aspects 1. reliability 2. quality sound. Is it that everybody forgoes reliability of the tubes (overvoltage) for sound or tube distortion ? Is it the idea that burning up tubes takes second place to finding that optimum circuit voltage ? maybe. The end result is that eveyones in the same train of thought which is, to hell with the proper voltages and if it sounds good who cares.

There still has'nt been anyone I have talked to who can tell me why its necessary to exceed published specs far beyond 20% accepted tolarance for tube circuits. Other than the fact that the more voltage applied to a tube produces more power- watts of output and loads more distortion. Which in most cases sounds very nice when its needed. However, do I need to drive the voltages up really high on the tubes to get nice clean tube sound. Probably not.

Has anyone here run a 5F1 with the published voltages on the Fender 5F1 schematic and had results that were less than expected. I have not heard anyone address this specifically. Although, I have run my amp with Fender
schematic voltages and experienced no disappointment in the sound. I had to use a transformer with lower B+ to acheive it. What I do not know at this point is how long will the tubes survive with one voltage (lower) compared to another (higher). And some really experienced people I have asked really don't know the answer to this thats why they avoid it or change the subject real fast or say "there are no tube specs, they are wrong ".

So , It always amazes me when you ask for a straight up answer to a spec and know one really knows it, wether it be a voltage on a schematic published by Fender or a maximum operating range spec found in RCA tube receiving manual. I just can't believe these figures were published as arbitrary standards. If you go beyond a design spec you should expect that device will suffer in some way , is this some part of what building guitar amps is all about. Is this why JJ current production tubes are designed with higher plate voltage ratings to deal with overvoltage due to mains voltage issues. On the same note, would you buy a output transformer that is rated for 10W when you know you'll be driving it at 20W. How much overvoltage to NOS tube can you put it through before you know its going to fail. I reason that the standards have changed on HT plate voltages due to the modern mains voltages which have changed considerably. This makes running a old original 5F1 into voltage regions it was not designed to handle when it was first produced. Could they have run the 5F1 with higher B+ if they wanted to ? , sure, but nothing Fender published in their schematics proves this. What benefit are current production tubes like JJ ? , they offer a solution to the elevated secondary voltages because the tubes have been designed with plate voltage specs to handle it, In return raising tube life expectancy. Can we say that the JJ plate voltage spec is wrong just like the RCA tube receiving manual or the Fender voltage spec on the original 5F1 schematic. This would not make any sense and this goes back to my original point that there are design specs and they should be used as a guide to building tube circuits like the 5F1 and many others.

not in any particular order, the balancing act is tube life, acceptable design voltages , quality of sound and components. The whole concept of voltages regarding the 5F1 is controversial, but given the above its also a design challenge.

[MWJB]

Voltages recommended in tube data sheets are conservative as the manufacturer does not know the specific application (& subsequent expected part life) the tube would be used in. Changing power tubes every couple of years on a hard used gigging amp is no big deal but in some applications the tubes might be run less hard to give several thousands of hours of use (car radios, home hi fi consoles, etc).

In the early days, amp manufacturers were more aware of following the data sheets, but amps had low power & poor fidelity...as players wanted more power & better cleans manufacturers (particularly Fender) pushed up voltages for better headroom...nothing broke...they kept doing it & nothing broke (don't forget that they only had to outlast the warranty, they weren't meant to last for decades - speakers were the big issue when it came to meeting power output & reliability)...until virtually every Fender amp made exceeds data sheet voltages.

Guys who build reproductions of classic amps for a living, generally get to hear good, original examples of type, they measure voltages, do A/B comparisons & go with the figures that sound best (it's all about the sound, with reasonable reliability) to them (that may be contemporary plate voltage, it may be less, it may be more).

When JJ, EH, Shuguang came to manufacturing 6V6s it was already apparent that if you were going to bias an amp to regular plate dissipations, that most of the existing amps were already running over voltage...if in the 1990's you were to a make a 6V6 with a max plate voltage of under 450vdc, it would be as much use as a chocolate fireguard - no one would have a use for it.

Considering Fender's Champ was made into the 1980's, you are not exactly using the latest available info on plate voltage & tube life by examining a late 50's design. Even at over 400vdc (to ground), the cheapest Chinese 6V6 will last several years in a champ...which sounds reasonable enough to me.