A Hampden County grand jury returned separate indictments charging the defendant with five aggra-[*636] voted rapes, one assault and battery by means of a dangerous weapon, and three incidents of assault and battery, all with respect to the same victim. After a jury trial in the Superior Court at which he did not testify, the defendant was found guilty of two aggravated rapes, three rapes as lesser included offenses, assault and battery by means of a dangerous weapon and two of the assault and battery charges. ^[1] He was acquitted on the remaining assault and battery indictment. The defendant moved for a new trial and requested an evi-dentiary hearing on the grounds that he had been deprived of his right to testify and that his trial counsel had been ineffective. He appeals from the denial of the motion without a hearing. ^[2]

Background

At approximately 11:00 p.m. on June 28, 1987, the victim and three friends arrived at Rumors Lounge in Springfield. During the evening she encountered Alexander Hall. They danced, then went out to the parking lot where they spent' some time in a car belonging to Jeanine Shepard, a friend of the victim. Eventually they joined the defendant, Darnell Freeman, who had accompanied Hall to Rumors. All three got into another car and drove off. During the ensuing hours, the victim was brutally beaten, raped repeatedly, and left naked and bleeding in a wooded area of Forest Park in Springfield.

[*637] The victim identified both the defendant and Hall as her assailants from a photographic array. Hall was indicted and pleaded guilty to one charge of rape. At the time of trial, he had not been sentenced. ^[3]

Trial Testimony

The victim and Hall both testified for the Commonwealth. They depicted the defendant as the primary aggressor who orchestrated the attack, beat the victim, and committed multiple sexual assaults. Hall admitted to a single instance of forced vaginal intercourse with the victim but depicted himself as a reluctant participant who tried to convince the defendant to release the victim throughout the attack.

While Hall’s and the victim’s testimony coincided in many respects, there were significant differences. The victim testified that, while she and Hall were in Jeanine Shepard’s car, they both drank rum and Hall smoked a marihuana cigarette. Hall maintained that he did not drink during that time. Hall further stated that the victim sat in the front seat of Shepard’s car with her legs across his lap and that he kissed her on the mouth. The victim, however, maintained that she and Hall did not kiss and that their bodies never touched while they were in Shepard’s car. Additionally, Hall claimed that after the defendant punched the victim in the face with his fist, causing her nose to bleed heavily, Hall attempted to stanch the bleeding. The victim, however, had no recollection of Hall thus coming to her aid.

The defendant did not present a case. He had given the police a written statement which was read to the jury as part of the Commonwealth’s case. His version of the night’s events was essentially contradictory of Hall’s and the victim’s. Hall was the aggressor, and the defendant went along reluctantly. The defendant admitted to a single instance of forced vaginal intercourse with the victim at Hall’s urging. At a later time, in order to assuage Hall, the defendant feigned intercourse with the victim.

[*638] The Motion for a New Trial

The defendant predicated his motion for a new trial and request for an evidentiary hearing on the claim that his trial counsel had erroneously advised him that, if he were to testify, his entire criminal record, including juvenile sex offenses, would be admissible at trial. For this reason, the defendant stated in his supporting affidavit, he did not testify in his own defense, despite his wish to tell his story to the jury. The defendant’s trial counsel, Mr. Edelmiro Martinez, Jr., submitted an affidavit stating that the defendant wanted to testify on his own behalf but that he was dissuaded from doing so by Martinez’s advice. Martinez counselled the defendant that it would be a “big mistake” for him to testify “in view of the statement he gave and signed to the police, and because the prosecution would be able to get before the jury his whole record, including the juvenile sex charges.”

The trial judge denied the motion without making specific findings concerning the credibility of the defendant’s claims. ^[4]

1. The admissibility of the defendant’s record for impeachment purposes. The defendant had been adjudicated delinquent on a charge of indecent assault and battery on a person under fourteen. The record of this adjudication would[*639] not have been admissible to impeach him. See G. L. c. 119, § 60. (Such an adjudication “shall not be lawful or proper evidence against such child for any purpose in any proceeding in any court . . . .”); Commonwealth v. A Juvenile (No. 2), 384 Mass. 390, 394 (1981) (a juvenile’s record is generally barred in a court proceeding in which the juvenile is a witness in the absence of “countervailing constitutional considerations”). Accord Commonwealth v. Ferrara, 368 Mass. 182, 185-186 (1975); Commonwealth v. Bembury, 406 Mass. 552, 556-561 (1990). Two charges which did not result in convictions were similarly inadmissible. ^[5] See Commonwealth v. Bishop, 296 Mass. 459, 461-462 (1937); Commonwealth v. Nassar, 351 Mass. 37, 44-45 (1966); Commonwealth v. Blaney, 387 Mass. 628, 637 (1982). The defendant’s prior adult convictions would have been admissible for impeachment under G. L. c. 233, § 21, if the trial judge properly determined that their probative value outweighed the danger of unfair prejudice. Commonwealth v. Reid, 400 Mass. 534, 536-541 (1987). ^[6] The defendant’s counsel did not move in limine or otherwise to bar the prosecutor from using the prior convictions for impeachment purposes. See Commonwealth v. Chase, 372 Mass. 736, 750-751 (1977).

2. The defendant’s right to testify on his own behalf. A criminal defendant has a fundamental right to testify on his own behalf. Rock v. Arkansas, 483 U.S. 44, 49-53 (1987). Commonwealth v. Siciliano, 19 Mass. App. Ct. 918, 920 (1984). Commonwealth v. Hennessey, 23 Mass. App. Ct. 384, 386 (1987). That right arises from several provisions of the United States Constitution. It is embodied in the due process clause of the Fourteenth Amendment, the compulsory process clause of the Sixth Amendment giving a defendant the right to call witnesses in his favor, and the Fifth[*640] Amendment’s guarantee against compelled self-incriminatian. Rock v. Arkansas, supra. 7

A right fundamental to a fair trial must be waived knowingly and intelligently. Schneckloth v. Bustamonte, 412 U.S. 218, 235-246 (1973). The waiver of a constitutional right “must be an intelligent act ‘done with sufficient awareness of the relevant circumstances and likely consequences.’ ” McMann v. Richardson, 397 U.S. 759, 766 (1970), quoting from Brady v. United States, 397 U.S. 742, 748 (1970). “A strict standard of waiver has been applied to those rights guaranteed to a criminal defendant to insure that he will be accorded the greatest possible opportunity to utilize every facet of the constitutional model of a fair criminal trial. Any trial conducted in derogation of that model leaves open the possibility that the trial reached an unfair result precisely because all the protections specified in the Constitution were not provided.” Schneckloth v. Bustamonte, supra at 241. See Commonwealth v. Waters, 399 Mass. 708, 716 (1987). The decision whether to testify is to be made personally by the defendant in consultation with his counsel. Commonwealth v. Waters, supra. Commonwealth v. Hennessey, supra, citing Wainwright v. Sykes, 433 U.S. 72, 93 n.1 (1977) (Burger, C.J., concurring); ABA Standards for Criminal Justice § 4.5.2(a) (2d ed. 1982).

If it was established that, as the defendant asserts in his affidavit, he would have testified in his own defense but for his counsel’s erroneous advice concerning the admissibility of his prior record, we would conclude that his waiver was invalid. The necessary factual determination cannot be made on the present record, however. Questions of credibility remain to be resolved by the motion judge. Commonwealth v. DeMinico, 408 Mass. 230, 244 (1990). An evidentiary hearing appears to be necessary. See Mass.R.Crim.P. 30(c)(3),[*641] 378 Mass. 901 (1979); Commonwealth v. Saarela, 15 Mass. App. Ct. 403, 406-407 (1983). Cf. Commonwealth v. Rosado, 408 Mass. 561, 568 (1990).

3. The burden of proof The defendant urges that, in determining whether he knowingly and intelligently waived his right to testify, the burden of proof should be placed on the Commonwealth. A defendant’s right to decide whether to testify on his own behalf is best protected if made without the government’s direct involvement. Because of the delicate balance between a defendant’s right to testify on his own behalf and his equally fundamental right not to testify, a trial judge should not be required to conduct a voir dire to determine whether a defendant has knowingly relinquished his right to testify. Such a colloquy might give the defendant the impression that he was being urged by the judge to testify and to waive his right not to testify. See Commonwealth v. Waters, supra at 716-717; Commonwealth v. Hennessey, supra at 388-390. ^[8] Here, the decision not to testify was made by the defendant in consultation with his counsel. The Commonwealth was not a party to the defendant’s decision-making process. The defendant, not the Commonwealth, has control over the relevant facts. He is, thus, better situated to prove his claim than is the Commonwealth to disprove it. See LaFave and Israel, Criminal Procedure § 10.3 (1984).

This is not a case of claimed deprivation of a constitutional trial right through government coercion. Contrast, e.g., Commonwealth v. Foster, 368 Mass. 100 (1975), interpreting Boykin v. Alabama, 395 U.S. 238, 242-244 (1969) (the government has the burden of showing, on the record, that a defendant has made a voluntary and intelligent guilty plea to determine, inter alla, “whether it is not being extracted from the defendant under undue pressure”). Here, the defendant,[*642] who is asserting “facts which were neither agreed upon nor apparent on the face of the [trial] record,” must prove those facts. Commonwealth v. Bertrand, 385 Mass. 356, 364 (1982), quoting from Commonwealth v. Bernier, 359 Mass. 13, 15 (1971). See Commonwealth v. Hubbard, 371 Mass. 160, 168 (1976) (the defendant, having pleaded guilty on a facially sufficient contemporaneous record, later claimed that at the time of his plea he had amnesia and therefore had not been competent to make the plea; the defendant had the burden of proving the facts he alleged). In order to succeed in his claim, the defendant must prove, by a preponderance of the evidence, that, but for his counsel’s erroneous advice concerning the admissibility of his probation record, he would have testified in his own defense. See Commonwealth v. Jordan, 207 Mass. 259, 275 (1911), aff'd, 225 U.S. 167 (1912) (the defendant, who moved for a new trial based on the allegation that a juror had been insane during his trial, had the burden of proving his claim by a preponderance of the evidence). Cf. Commonwealth v. Clifford, 374 Mass. 293, 303 (1978) (the defendant must show “by a preponderance of the evidence” that procedures used in a lineup were impermissibly suggestive). See LaFave, Search and Seizure § 11.2(c)(2d ed. 1987) (in the context of Fourth Amendment suppression hearings, where the burden of proof is on the defendant, courts do not usually discuss the applicable standard of proof; “[w]hen mentioned, it is said to be the preponderance standard, and there is no indication of any effort to impose a higher burden on defendants [a change which would seem indefensible]”).

If the defendant’s waiver is determined to be invalid, he will be entitled to a new trial. We need not decide whether a harmless error analysis, see Chapman v. California, 386 U.S. 18, 20-24 (1967), ^[9] could ever be applicable to a criminal defendant’s invalid waiver of his right to testify on his own be[*643] half. ^[10] In this case, we cannot say that an invalid waiver would have been harmless beyond a reasonable doubt. The defendant had no case other than his critically different version of the events. While the prosecution presented a strong case, there were significant differences in the testimony of the two main prosecution witnesses, Hall and the victim. Moreover, Hall had a motive for placing the lion’s share of the blame on the defendant. He had pleaded guilty to one count of rape, h^d not yet been sentenced, and had additional indictments pending. The victim testified that she had been “a little drunk” when she left Rumors Lounge with Hall and the defendant. The assaults took place in a car and in a dark wooded area during the night, and the victim apparently had not previously been acquainted with either of her attackers.

Finally, the jury did not wholly accept the Commonwealth’s case. They acquitted the defendant of one assault and battery charge and found him guilty of the lesser included offenses of simple rape in three of the aggravated rape charges. We cannot say that the defendant’s personal testimony (with all that it entails), which it is fair to assume would have been essentially consistent with the statement he gave to the police, could not have at least raised reasonable doubt in the minds of the jurors as to the identity of the principal offender.

We do not address the defendant’s claim of ineffective assistance of counsel on the basis of incorrect legal advice (a theory which in the ultimate analysis involves harmless error consideration ^[11] ) because a decision regarding the validity of[*644] his waiver will be dispositive. Compare Commonwealth v. Rosado, 408 Mass. at 568. If the waiver is found to be valid, as having been knowing and intelligent, then the the defendant would not have been deprived of an otherwise available substantial ground of defense. See Commonwealth v. Saferian, 366 Mass. 89, 96 (1974). If the waiver is determined to be invalid, he will be entitled to a new trial.

The order denying the motion for a new trial is reversed, and the case is remanded to the Superior Court for further proceedings before the trial and motion judge consistent with this opinion.

So ordered.